docs: create v26, fold v25.x into a single version

[thymikee]

Summary

Updating documentation for the upcoming v26. Folded multiple 25 minor versions into a single 25.x.

Test plan

Local, clicked through some core pages and seems to work as expected.

[codecov-io]

Codecov Report

Merging #9956 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #9956   +/-   ##
=======================================
  Coverage   64.06%   64.06%           
=======================================
  Files         293      293           
  Lines       12457    12457           
  Branches     3067     3069    +2     
=======================================
  Hits         7981     7981           
  Misses       3833     3833           
  Partials      643      643           

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[SimenB]

Reviewing on mobile crashes chrome, but I assume it's good 😃 need to make sure any doc updates get in here as well

[cpojer]

👍

[SimenB]

Does this come with the updated docs for fake timers? Only doc change I know of after this was opened

[thymikee]

Yup, I rebased it recently and it's there

[github-actions]

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