docs: document the current_py_cc_headers and libraries targets (#1927)

This come from the recent slack thread where someone was trying to
figure out how to get at the runtime's headers.

docs/sphinx/api/python/cc/index.md

diff --git a/docs/sphinx/api/python/cc/index.md b/docs/sphinx/api/python/cc/index.md
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+:::{bzl:currentfile} //python/cc:BUILD.bazel
+:::
+# //python/cc
+
+:::{bzl:target} current_py_cc_headers
+
+A convenience target that provides the Python headers. It uses toolchain
+resolution to find the headers for the Python runtime matching the interpreter
+that will be used. This basically forwards the underlying
+`cc_library(name="python_headers")` target defined in the `@python_X_Y` repo.
+
+This target provides:
+
+* `CcInfo`: The C++ information about the Python headers.
+:::
+
+:::{bzl:target} current_py_cc_libs
+
+A convenience target that provides the Python libraries. It uses toolchain
+resolution to find the libraries for the Python runtime matching the interpreter
+that will be used. This basically forwards the underlying
+`cc_library(name="libpython")` target defined in the `@python_X_Y` repo.
+
+This target provides:
+
+* `CcInfo`: The C++ information about the Python libraries.
+:::