Document guidance on type annotations - #1939

Closes #1304

Summary

• Document CPython's current practice for type annotations in standard-library modules.
• Explain the role of typeshed, acknowledge existing annotated modules and tools, and distinguish source annotations from user-facing documentation.
• Link to the core development discussion that records the rationale and current state.

Validation

• Read the rendered page in the successful Read the Docs preview build.
• pre-commit.ci passed.
• git diff --check passed.
• The local Windows docs build could not install the full pinned requirements: sphinx_linklint was unavailable from the package index and Sphinx 9.1+ requires Python 3.12, while the available interpreters are Python 3.11 and 3.14. The hosted docs build succeeded.