Consider fast-tracking doc-only PRs

[jodh-intel]

Idea: we could get Jenkins to check if a PR only has doc changes on it and just "return success" to avoid chewing CPU time for no reason: we already additionally check master so there seems little point running the full test suite for non-code changes.

[grahamwhaley]

Something I got from another project - was you could add a special tag (in the other project I think it was something in the commit - but - I suspect a github label may also work) - where you could notify it was a docs change, and it would skip the CI builds. Just throwing that in as an option of how to tag PRs as not needing full builds.
/cc @sboeuf and @chavafg for jenkins and CI scrtipt thinking...