`Doc/c-api/init.rst` is way too big

[ZeroIntensity]

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.).

[StanFromIreland]

These moves are quite unfortunate for translations, but I’ll try to keep track of this and inform translation teams. Are you going to be backporting to 3.14?

[ZeroIntensity]

That would be ideal. Otherwise, any changes to the docs of those APIs will have conflicts when backporting. Is there anything that can be done to improve this for translations?

[vstinner]

I think we should split this up into two pages.

It could be 3 pages:

• Interpreter initialization and finalization
• Threads and concurrency
• Subinterpreters

[StanFromIreland]

Is there anything that can be done to improve this for translations?

Automatically, for non-Transifex translations (TX memory should merge them back in), no. It will require each translation team to manually merge the messages back in from an older commit. Due to this, I want to notify them ASAP after merge to recover the message and not re-translate things. I'll look into adding a guide for it to the devguide.

[ZeroIntensity]

It could be 3 pages

Sounds good. I'll work on this once #141810 is done.