doc: document current_cc_headers and related targets

This is a ticket to document the targets in https://github.com/bazel-contrib/rules_python/blob/cab415d82ebbfd1b8b2d81ef61baf3f9a274a1b2/python/cc/BUILD.bazel#L15 and other in the same file so that we can point users to documentation on how to use toolchain lookup instead of the legacy alias targets in toolchain repos that we are still creating.

Maybe this ticket can also document similar targets for the python toolchain as well.

EDIT: Looked at the docs again and I see that the easiest way to document this would be to add a sentence or two in https://rules-python.readthedocs.io/en/latest/toolchains.html#python-c-toolchain-type so that the users know how to consume headers and `libpython`.

Cross-referencing https://rules-python.readthedocs.io/en/latest/toolchains.html#local-toolchain with `seealso` to the `python-c-toolchain-type` would be also useful.

python/cc/BUILD.bazel のターゲットを読み、その後 toolchains.html の「python-c-toolchain-type」セクションと「local-toolchain」セクションを確認してください。ユーザーが toolchain の検索を通じて headers と libpython をどのように利用するかを文書化し、提案されている seealso の相互参照を追加してください。類似する Python toolchain ターゲットも対象に含めるべきかどうかを明確にしてください。

索引モデルが issue の本文から書いたものです。