docs: highlight optional CPython build dependencies - #1975

Summary

• Added a note near the beginning of the "Setup and Building" guide to improve the visibility of optional CPython build dependencies.
• Included a reference to the existing dependency installation documentation.
• Clarified that these additional libraries are needed for certain optional standard-library extension modules, but not for a basic build.

Testing

• .\make.bat html — Passed.
• .\make.bat check — Passed.
• git diff --check — Passed.
• .\make.bat linkcheck — Not completed due to rate limiting from the Internet Archive's Wayback Machine.

Related issue

Refs #1338