Skip github CI/CD workflows for markdown and cff files

[dennisvang]

Changes to markdown or cff (citation) files do not require CI/CD workflows to run.

[dennisvang]

This should be so simple, but, apparently, it is not: actions/runner#2324

The idea was to prevent CI runs for PR updates (commits added to a PR topic branch) that only change markdown and other text files.

However, apparently, the pull_request synchronize activity considers all the changes in the PR, not just those in the latest commit. That means if the entire PR contains any changes to other file types, CI will still trigger, even if the latest commit only changes files that match paths-ignore.

Still, it should work for PRs that only change files mentioned in paths-ignore.

For individual commits in PRs that also change other file types, we'll still need to use [skip-ci] in the commit message, which is too easy to forget...