ci: allow manual trigger of doc build workflow

[hadrien]

Summary

• Add workflow_dispatch trigger to the doc build & publish workflow so it can be run on demand from the GitHub Actions UI

Test plan

• After merge, trigger the workflow manually from the Actions tab and verify it builds and deploys docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.36%. Comparing base (c1f0463) to head (b001820).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  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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