docs(gazelle): Migrate Gazelle docs to ReadTheDocs, part 5/5: target types (#3147)

Part of #3082

5th of ~~5~~ 6 PRs.

+ Migrate target types docs from `gazelle/README.md` to
`gazelle/docs/installation_and_usage.md`
+ `Libraries` section:
  + Update wording
  + list for the generation mode types
+ `Binaries` section:
  + Slight rewording
  + `note` block instead of simple "Note that ..."
+ Mechanical updates:
  + Wrap at ~80 chars
  + Use MyST directives and roles.

gazelle/README.md

diff --git a/gazelle/README.md b/gazelle/README.md
index 4de2c3c..30067b4 100644
--- a/gazelle/README.md
+++ b/gazelle/README.md
@@ -6,76 +6,6 @@
 :::
 
 
-### Libraries
-
-Python source files are those ending in `.py` but not ending in `_test.py`.
-
-First, we look for the nearest ancestor BUILD file starting from the folder
-containing the Python source file.
-
-In package generation mode, if there is no `py_library` in this BUILD file, one
-is created using the package name as the target's name. This makes it the
-default target in the package. Next, all source files are collected into the
-`srcs` of the `py_library`.
-
-In project generation mode, all source files in subdirectories (that don't have
-BUILD files) are also collected.
-
-In file generation mode, each file is given its own target.
-
-Finally, the `import` statements in the source files are parsed, and
-dependencies are added to the `deps` attribute.
-
-### Unit Tests
-
-A `py_test` target is added to the BUILD file when gazelle encounters
-a file named `__test__.py`.
-Often, Python unit test files are named with the suffix `_test`.
-For example, if we had a folder that is a package named "foo" we could have a Python file named `foo_test.py`
-and gazelle would create a `py_test` block for the file.
-
-The following is an example of a `py_test` target that gazelle would add when
-it encounters a file named `__test__.py`.
-
-```starlark
-py_test(
-    name = "build_file_generation_test",
-    srcs = ["__test__.py"],
-    main = "__test__.py",
-    deps = [":build_file_generation"],
-)
-```
-
-You can control the naming convention for test targets by adding a gazelle directive named
-`# gazelle:python_test_naming_convention`.  See the instructions in the section above that
-covers directives.
-
-### Binaries
-
-When a `__main__.py` file is encountered, this indicates the entry point
-of a Python program. A `py_binary` target will be created, named `[package]_bin`.
-
-When no such entry point exists, Gazelle will look for a line like this in the top level in every module:
-
-```python
-if __name == "__main__":
-```
-
-Gazelle will create a `py_binary` target for every module with such a line, with
-the target name the same as the module name.
-
-If `python_generation_mode` is set to `file`, then instead of one `py_binary`
-target per module, Gazelle will create one `py_binary` target for each file with
-such a line, and the name of the target will match the name of the script.
-
-Note that it's possible for another script to depend on a `py_binary` target and
-import from the `py_binary`'s scripts. This can have possible negative effects on
-Bazel analysis time and runfiles size compared to depending on a `py_library`
-target. The simplest way to avoid these negative effects is to extract library
-code into a separate script without a `main` line. Gazelle will then create a
-`py_library` target for that library code, and other scripts can depend on that
-`py_library` target.
-
 ## Developer Notes
 
 Gazelle extensions are written in Go.

gazelle/docs/installation_and_usage.md

diff --git a/gazelle/docs/installation_and_usage.md b/gazelle/docs/installation_and_usage.md
index e764957..123f30a 100644
--- a/gazelle/docs/installation_and_usage.md
+++ b/gazelle/docs/installation_and_usage.md
@@ -149,3 +149,86 @@
 
 That's it, now you can finally run `bazel run //:gazelle` anytime
 you edit Python code, and it should update your `BUILD` files correctly.
+
+
+## Target Types and How They're Generated
+
+### Libraries
+
+Python source files are those ending in `.py` that are not matched as a test
+file via the `# gazelle:python_test_file_pattern` directive. By default,
+python source files are all `*.py` files except for `*_test.py` and
+`test_*.py`.
+
+First, we look for the nearest ancestor `BUILD(.bazel)` file starting from
+the folder containing the Python source file.
+
++ In `package` generation mode, if there is no {bzl:obj}`py_library` in this
+  `BUILD(.bazel)` file, one is created using the package name as the target's
+  name. This makes it the default target in the package. Next, all source
+  files are collected into the `srcs` of the {bzl:obj}`py_library`.
++ In `project` generation mode, all source files in subdirectories (that don't
+  have `BUILD(.bazel)` files) are also collected.
++ In `file` generation mode, each python source file is given its own target.
+
+Finally, the `import` statements in the source files are parsed and
+dependencies are added to the `deps` attribute of the target.
+
+
+### Tests
+
+A {bzl:obj}`py_test` target is added to the `BUILD(.bazel)` file when gazelle
+encounters a file named `__test__.py` or when files matching the
+`# gazelle:python_test_file_pattern` directive are found.
+
+For example, if we had a folder that is a package named "foo" we could have a
+Python file named `foo_test.py` and gazelle would create a {bzl:obj}`py_test`
+block for the file.
+
+The following is an example of a {bzl:obj}`py_test` target that gazelle would
+add when it encounters a file named `__test__.py`.
+
+```starlark
+py_test(
+    name = "build_file_generation_test",
+    srcs = ["__test__.py"],
+    main = "__test__.py",
+    deps = [":build_file_generation"],
+)
+```
+
+You can control the naming convention for test targets using the
+`# gazelle:python_test_naming_convention` directive.
+
+
+### Binaries
+
+When a `__main__.py` file is encountered, this indicates the entry point
+of a Python program. A {bzl:obj}`py_binary` target will be created, named
+`[package]_bin`.
+
+When no such entry point exists, Gazelle will look for a line like this in
+the top level in every module:
+
+```python
+if __name == "__main__":
+```
+
+Gazelle will create a {bzl:obj}`py_binary` target for every module with such
+a line, with the target name the same as the module name.
+
+If the `# gazelle:python_generation_mode` directive is set to `file`, then
+instead of one {bzl:obj}`py_binary` target per module, Gazelle will create
+one {bzl:obj}`py_binary` target for each file with such a line, and the name
+of the target will match the name of the script.
+
+:::{note}
+It's possible for another script to depend on a {bzl:obj}`py_binary` target
+and import from the {bzl:obj}`py_binary`'s scripts. This can have possible
+negative effects on Bazel analysis time and runfiles size compared to
+depending on a {bzl:obj}`py_library` target. The simplest way to avoid these
+negative effects is to extract library code into a separate script without a
+`main` line. Gazelle will then create a {bzl:obj}`py_library` target for
+that library code, and other scripts can depend on that {bzl:obj}`py_library`
+target.
+:::