docs: document current_py_cc_headers and related toolchain targets

[Saish-3]

Adds documentation explaining:

• current_py_cc_headers
• current_py_cc_headers_abi3
• current_py_cc_libs

Describes how these targets expose headers and libraries
from the active Python C toolchain and clarifies that they
should be used instead of legacy alias targets when embedding
Python or building C extensions.

This addresses issue #3098.

[gemini-code-assist]

Summary of Changes

Hello @Saish-3, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the rules_python documentation by introducing a new section that explains the purpose and usage of current_py_cc_headers, current_py_cc_headers_abi3, and current_py_cc_libs. These targets are crucial for correctly consuming Python C headers and libraries from the active Python C toolchain, especially when embedding Python or developing C extensions, thereby improving compatibility and clarity for users.

Highlights

• Documentation of current_py_cc_headers: Added documentation for @rules_python//python/cc:current_py_cc_headers, explaining its role in exposing Python C toolchain headers.
• Documentation of current_py_cc_headers_abi3: Added documentation for @rules_python//python/cc:current_py_cc_headers_abi3, detailing its function in providing ABI3-compatible Python C toolchain headers.
• Documentation of current_py_cc_libs: Added documentation for @rules_python//python/cc:current_py_cc_libs, clarifying its use for exposing Python C toolchain libraries.
• Usage guidance for Python C toolchain targets: Provided guidance on using these new targets instead of legacy aliases for embedding Python or building C extensions, ensuring compatibility.
• Toolchain registration requirements: Explained the necessity of registering a Python C toolchain for these targets to function, noting automatic registration under bzlmod and explicit registration for non-bzlmod setups.

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Changelog

• docs/toolchains.md
  • Added a new section titled "Consuming Python C headers and libraries" to document the current_py_cc_headers, current_py_cc_headers_abi3, and current_py_cc_libs targets.
  • Explained the behavior of these targets as cc_library-like entities forwarding providers from the underlying toolchain.
  • Included instructions on toolchain registration requirements for these targets.
  • Provided guidance on using these targets over legacy aliases for C extensions and Python embedding.

Activity

• No activity has occurred on this pull request since its creation.

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[gemini-code-assist]

Code Review

This pull request adds valuable documentation for current_py_cc_headers and related targets, explaining their purpose and usage. The new section is clear and well-written. I've provided a few minor suggestions to improve formatting consistency with the rest of the document by wrapping target names and keywords in backticks.

[gemini-code-assist]

For consistency with the rest of the document, please wrap bzlmod and non-bzlmod in backticks to format them as code.

Suggested change

	Under bzlmod, a toolchain is registered automatically. In non-bzlmod
	setups, users must ensure that a toolchain is explicitly registered.
	Under `bzlmod`, a toolchain is registered automatically. In `non-bzlmod`
	setups, users must ensure that a toolchain is explicitly registered.