ci: extract reusable docs workflow, add manual docs-publish trigger

[petercorke]

Summary

• Extracts the Sphinx + JupyterLite build/deploy steps out of ci.yml into a workflow_call reusable workflow (docs.yml), so the logic lives in one place.
• Adds docs-publish.yml, a workflow_dispatch-only entry point that calls the same docs.yml steps without waiting on the full OS/Python test matrix — useful for a docs-only fix that doesn't need the whole suite to re-run first.
• ci.yml behavior is unchanged: its docs job still runs only after test-core/test pass, just delegates to docs.yml instead of duplicating the steps.
• Deployment to GitHub Pages still only happens when run against main (checked inside docs.yml's docs-deploy job), so a manual dispatch from another branch just builds, it doesn't publish.

Test plan

• Confirm ci.yml still builds + deploys docs on a push to main, same as before
• Manually dispatch "Docs Publish (manual)" from the Actions tab against main and confirm it builds and deploys the site
• Manually dispatch it from a non-main branch and confirm it builds but does not attempt to deploy

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (4ba11d1) to head (b1ac118).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #658   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        143     143           
  Lines      14050   14050           
=====================================
  Misses     14050   14050           

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