docs: build doc on tags instead of main

[hadrien]

Summary

• Trigger doc build & publish workflow on tag pushes instead of pushes to main
• Keep docs/** branch trigger for previewing doc changes

Test plan

• Push a tag and verify the doc workflow runs and deploys to GitHub Pages

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.36%. Comparing base (8df1378) to head (bbba233).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #58   +/-   ##
=======================================
  Coverage   97.36%   97.36%           
=======================================
  Files           1        1           
  Lines         114      114           
  Branches        2        2           
=======================================
  Hits          111      111           
  Misses          3        3           

Flag	Coverage Δ
fastapi-post121	97.36% <ø> (ø)
fastapi-pre121	97.36% <ø> (ø)
sqlmodel	97.36% <ø> (ø)

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