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.

阅读 python/cc/BUILD.bazel 中的 targets，然后查看 toolchains.html 的“python-c-toolchain-type”和“local-toolchain”部分。记录用户如何通过 toolchain 查找来使用 headers 和 libpython，并添加建议的 seealso 交叉引用；明确是否也应涵盖类似的 Python toolchain targets。

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