gh-128642: Add free threading glossary of building blocks to InternalDocs

[overlorde]

Summary

• Adds InternalDocs/free_threading.md — a glossary of the low-level building blocks used in free-threaded CPython
• Covers: object header layout, biased reference counting, critical sections, PyMutex, immortalization, deferred refcounting, GC bit flags, atomic operations, and QSBR (cross-reference)
• Adds link to the new document in InternalDocs/README.md

Closes gh-128642.