Only build & deploy pkgdown on release

[kamilzyla]

Changes

Closes #527:

1. Trigger our pkgdown.yml workflow on release instead of push. This way our website will show docs for the latest release instead of development version.
2. Document the change.

This should resolve our immediate problem of confusing users. Ideally we'd allow users select the documentation version to browse, but I haven't found any simple way to achieve it. I have summarized my research in a new issue: #530.

How to test

We'll see if this works as expected when we release Rhino 1.6.

If needed, we can manually trigger the workflow for any tag/branch. I used this to deploy v1.5 docs now. I used a temporary branch (tmp-docs-1.5) instead of v1.5.0 tag to include the CSS fix from #517.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6926a37) 17.82% compared to head (5983bce) 17.82%.

❗ Current head 5983bce differs from pull request most recent head 0ea8aa6. Consider uploading reports for the commit 0ea8aa6 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #529   +/-   ##
=======================================
  Coverage   17.82%   17.82%           
=======================================
  Files           9        9           
  Lines         404      404           
=======================================
  Hits           72       72           
  Misses        332      332           

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[jakubnowicki]

LGTM