DM-40883: Add v26 release notes

[timj]

Checklist

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

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.06% ⚠️

Comparison is base (870df52) 83.52% compared to head (cd9526c) 83.46%.

Additional details and impacted files

@@             Coverage Diff             @@
##           v26.0.x     #375      +/-   ##
===========================================
- Coverage    83.52%   83.46%   -0.06%     
===========================================
  Files           77       77              
  Lines         9212     9212              
  Branches      1782     1782              
===========================================
- Hits          7694     7689       -5     
- Misses        1227     1233       +6     
+ Partials       291      290       -1     

see 1 file with indirect coverage changes

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