Update GC docs for incremental collection.

[markshannon]

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📚 Documentation preview 📚: https://cpython-devguide--1379.org.readthedocs.build/

[markshannon]

Thanks for the comments and suggestions.

[mr-bronson]

Please note the inconsistency in argument or variable names:

This devguide speaks of threshold_0 and threshold_1 (note the underscore), but also threshold2 (no underscore). If the aim is to match the main gc documentation, the underscores should be omitted.

[pablogsal]

Let's hold until we figure out https://discuss.python.org/t/incremental-gc-and-pushing-back-the-3-13-0-release/65285

[willingc]

Garbage collection doc has been moved to the CPython repo. Unless there is an objection, I recommend closing this devguide issue.

[markshannon]

All these docs are now in main