Add COSMOS 6 upgrade guide

[ryan-pratt]

closes #1744

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.59%. Comparing base (f0193f4) to head (3495613).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1754      +/-   ##
==========================================
- Coverage   79.59%   79.59%   -0.01%     
==========================================
  Files         517      517              
  Lines       40713    40713              
==========================================
- Hits        32406    32404       -2     
- Misses       8307     8309       +2     

Flag	Coverage Δ
python	84.30% <ø> (-0.02%)	⬇️
ruby-api	48.64% <ø> (ø)
ruby-backend	82.60% <ø> (ø)

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[jmthomas]

Made a few updates to the guide and regenerated the site

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud