docs: document compiler migration paths

[aryansk]

Summary

• document the migration from distutils.ccompiler to the standalone compilers.C API
• add mappings for Compiler, new_compiler, get_default_compiler, and show_compilers
• document compilers.platform.get_platform as the replacement for distutils.util.get_platform
• clarify that setuptools._distutils is private and that Setuptools does not provide a re-export layer

Resolves #5272

Validation

• git diff --check
• checked the documented names against the compiler package layout and the public API issue linked from Fill migration-guide gaps for ccompiler / new_compiler / get_platform #5272
• full documentation build not run locally because this sparse checkout does not include installed documentation dependencies

AI disclosure

This draft was prepared with AI assistance. No claim of human review is made; it remains a draft for maintainer review.