Documentation

• Expand the "what's new" section a bit.
• Update Doc/library/gc.rst, speciifically how the thresholds work now.
• Update the dev guide to explain how incremental collection works.

Linked PRs

• GH-117759: Document incremental GC #123266
• [3.13] GH-117759: Document incremental GC (GH-123266) #123395
• GH-117759: Update GC docs for incremental collection #126695