Documentation

While working on #141810, I noticed that init.rst covers three very complicated topics:

1. Interpreter initialization and finalization
2. Threads and concurrency
3. Subinterpreters

This makes it pretty huge and difficult to find things. I think we should split this up into two pages. Let's have a page for interpreter-related topics (initialization, finalization, subinterpreters), and a separate page for anything related to threads (PyMutex, critical sections, thread states, etc.).

Linked PRs

• gh-141811: Split up init.rst into multiple pages #144844
• [3.14] gh-141811: Split up init.rst into multiple pages (GH-144844) #145061