Docs: move deprecations into include files

[hugovk]

Documentation

Re: https://discuss.python.org/t/streamline-whats-new-by-moving-deprecations-and-removals-out-of-news/53997/8

To avoid needing to duplicate and sync the deprecation sections across What's New files, and ease backports, move them into include files.

• Python:
  • Use include files for whatsnew/3.12.rst deprecations
  • Use include files for whatsnew/3.13.rst deprecations
  • Use include files for whatsnew/3.14.rst deprecations
• C API:
  • Use include files for deprecations
• Create dedicated page to list deprecations using include files

Linked PRs

• gh-122085: Use include files for whatsnew/3.13.rst deprecations #121241
• [3.13] gh-122085: Use include files for whatsnew/3.13.rst deprecations (GH-121241) #122038
• gh-122085: Use include files for whatsnew/3.12.rst deprecations #122093
• [3.13] gh-122085: Use include files for whatsnew/3.12.rst deprecations (GH-122093) #122223
• [3.13] gh-122085: Use include files for whatsnew/3.12.rst deprecations, including 3.16 (GH-122093) #122225
• [3.12] gh-122085: Use include files for whatsnew/3.12.rst deprecations (GH-122093) #122224
• gh-122085: Use include files for whatsnew/3.14.rst deprecations #122242
• [3.13] gh-122085: Use include files for whatsnew/3.14.rst deprecations (GH-122242) #122350
• [3.12] gh-122085: Use include files for whatsnew/3.14.rst deprecations (GH-122242) #122351
• gh-122085: Use include files for C API deprecations #109843
• gh-122085: Create dedicated page for deprecations #122352
• [3.13] gh-122085: Create dedicated page for deprecations (GH-122352) #122374
• [3.12] gh-122085: Create dedicated page for deprecations (GH-122352) #122375
• [3.13] GH-122085: Use include files for C API deprecations (GH-109843) #122422
• [3.12] GH-122085: Use include files for C API deprecations (GH-109843) #122423