CI: When only Hugo docs change, other workflows do not need to run

[bernhardkaindl]

Optional improvement for fast CI completion when only Hugo docs change on push/PRs:

When only Hugo docs change on push and PRs, other workflows can be skipped using
paths-ignore: for docs/** and .github/workflows/hugo.yml to not trigger other
workflows if only these paths change when pushing to a branch / work on a PR.

This is completely optional, but would be nice to have when only working on Hugo docs.
It does not change scheduled CI runs. It changes only on-demand push/PR workflow runs.

[psafont]

We may not want to merge this if we want the new docs to have content autogenerated from the API

[lindig]

If we have code changes that also change documentation, the code-related workflows would still trigger the CI. So I am not sure if @psafont's concern is a reason to not merge this. We also have the cron builds; so would be willing to try this

[bernhardkaindl]

We may not want to merge this if we want the new docs to have content autogenerated from the API

If we have code changes that also change documentation, the code-related workflows would still trigger the CI. So I am not sure if @psafont's concern is a reason to not merge this. We also have the cron builds; so would be willing to try this

Indeed, Christian is correct.

And actually, this PR is not about autogenerating API docs:
This would need to be done as part of .github/workflows/hugo.yml.
This this PR does not change hugo.yml at all.

This PR is about the reverse direction:

It is about that the docs do not change to code:

As docs have no effect on the code, the code CI does not need to run when only docs were changed.