Docs-only PRs don't trigger docs CI

[TomNicholas]

I noticed in https://github.com/zarr-developers/VirtualiZarr/pull/746 that a docs-only PR doesn't trigger the CI which builds the documentation. It makes sense for a docs-only PR to not run the test suite, but it would be nice if it did trigger the docs build.

[TomNicholas]

Okay now it did trigger? I'm confused what happened there.

[maxrjones]

RTD community has build limits - https://docs.readthedocs.com/platform/stable/builds.html#build-resources. It's possible that we already had two concurrent builds and had to wait for those to finish before a new build was triggered.