DM-40883 (main): Add v26 release notes

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (d68f275) 78.70% compared to head (20cd7a1) 78.70%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   78.70%   78.70%           
=======================================
  Files          40       40           
  Lines        3090     3090           
  Branches      519      519           
=======================================
  Hits         2432     2432           
  Misses        568      568           
  Partials       90       90           

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