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.