Automatically add [ci-skip] to documentation related commits

[DirtyF]

I always forgot to add [ci-skip] to documentation commits to avoid triggering a useless CI build. 😕

Could @jekyllbot help?

Given a contributor update Jekyll's website or documentation
When the PR is submitted
Then all commits prefixed with `Docs:` do not trigger a CI check
And all modified files in the `_docs` directory do not trigger a CI check
And all PR tagged with `documentation`  do not trigger a CI check
And all PR tagged with `site`  do not trigger a CI check

[ashmaroli]

On thinking about this, I believe there's nothing much JekyllBot could do here. CI builds are immediately triggered by git push. So the [ci skip] attribute would only make sense if its present before the git push event.

A similar approach is already followed in practice when JekyllBot updates the History file, irrespective of the changes: