docs(gazelle): Migrate Gazelle docs to ReadTheDocs, part 6/5: development (#3149)

Fixes #3082

6th of 5 PRs.

+ Migrate gazelle development docs from `gazelle/README.md` to
`gazelle/docs/development.md`
+ Add information on writing tests
+ 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 30067b4..128fb1f 100644
--- a/gazelle/README.md
+++ b/gazelle/README.md
@@ -1,22 +1,6 @@
 # Python Gazelle plugin
 
 :::{note}
-The gazelle plugin docs are being migrated to our primary documentation on
+The gazelle plugin docs have been migrated to our primary documentation on
 ReadTheDocs. Please see https://rules-python.readthedocs.io/gazelle/docs/index.html.
 :::
-
-
-## Developer Notes
-
-Gazelle extensions are written in Go.
-See the gazelle documentation https://github.com/bazelbuild/bazel-gazelle/blob/master/extend.md
-for more information on extending Gazelle.
-
-If you add new Go dependencies to the plugin source code, you need to "tidy" the go.mod file.
-After changing that file, run `go mod tidy` or `bazel run @go_sdk//:bin/go -- mod tidy`
-to update the go.mod and go.sum files. Then run `bazel run //:gazelle_update_repos` to have gazelle
-add the new dependenies to the deps.bzl file. The deps.bzl file is used as defined in our /WORKSPACE
-to include the external repos Bazel loads Go dependencies from.
-
-Then after editing Go code, run `bazel run //:gazelle` to generate/update the rules in the
-BUILD.bazel files in our repo.

gazelle/docs/development.md

diff --git a/gazelle/docs/development.md b/gazelle/docs/development.md
new file mode 100644
index 0000000..29ac7a0
--- /dev/null
+++ b/gazelle/docs/development.md
@@ -0,0 +1,57 @@
+# Development
+
+Gazelle extensions are written in Go.
+
+See the [Gazelle documentation][gazelle-extend] for more information on
+extending Gazelle.
+
+[gazelle-extend]: https://github.com/bazel-contrib/bazel-gazelle/blob/master/extend.md
+
+
+## Dependencies
+
+If you add new Go dependencies to the plugin source code, you need to "tidy"
+the go.mod file.  After changing that file, run `go mod tidy` or
+`bazel run @go_sdk//:bin/go -- mod tidy` to update the `go.mod` and `go.sum`
+files. Then run `bazel run //:gazelle_update_repos` to have gazelle add the
+new dependencies to the `deps.bzl` file. The `deps.bzl` file is used as
+defined in our `/WORKSPACE` to include the external repos Bazel loads Go
+dependencies from.
+
+Then after editing Go code, run `bazel run //:gazelle` to generate/update
+the rules in the `BUILD.bazel` files in our repo.
+
+
+## Tests
+
+:::{seealso}
+{gh-path}`gazelle/python/testdata/README.md`
+:::
+
+To run tests, {command}`cd` into the {gh-path}`gazelle` directory and run
+`bazel test //...`.
+
+Test cases are found at {gh-path}`gazelle/python/testdata`. To make a new
+test case, create a directory in that folder with the following files:
+
++ `README.md` with a short blurb describing the test case(s).
++ `test.yaml`, either empty (with just the docstart `---` line) or with
+  the expected `stderr` and exit codes of the test case.
++ and empty `WORKSPACE` file
+
+You will also need `BUILD.in` and `BUILD.out` files somewhere within the test
+case directory. These can be in the test case root, in subdirectories, or
+both.
+
++ `BUILD.in` files are populated with the "before" information - typically
+  things like Gazelle directives or pre-existing targets. This is how the
+  `BUILD.bazel` file looks before running Gazelle.
++ `BUILD.out` files are the expected result after running Gazelle within
+  the test case.
+
+:::{tip}
+The easiest way to create a new test is to look at one of the existing test
+cases.
+:::
+
+The source code for running tests is {gh-path}`gazelle/python/python_test.go`.

gazelle/docs/index.md

diff --git a/gazelle/docs/index.md b/gazelle/docs/index.md
index 6758e11..f276b0c 100644
--- a/gazelle/docs/index.md
+++ b/gazelle/docs/index.md
@@ -45,4 +45,5 @@
 installation_and_usage
 directives
 annotations
+development
 ```