For readers -- split up `init.rst`

[ZeroIntensity]

• Link to discussion about the proposal (Discourse or GitHub): Doc/c-api/init.rst is way too big

I would like to split up the init.rst file in the C API documentation into three separate pages:

1. Interpreter lifecycle (initialization and finalization).
2. Concurrency (the GIL, thread states, synchronization).
3. Subinterpreters (Py_NewInterpreter*, PyInterpreterConfig) + PyInterpreterState* APIs.

We might even want an additional page for profiling (PyRefTracer and PyTrace*).

I've heard pushback because this would break external references to the existing items on that page.