[CI doc] safely testing experimental CI features
After causing a few CI workflow disruptions with my recent attempts to figure out how to get circleCI do something new (skip heavy builds on doc-only PRs) I realized that future such experiments can be much smoother and lead to close to zero annoyance to anybody involved in submitting and handling PRs.

This PR documents my idea on how to do it given the current limitations of CircleCI and GithubActions, so that we could continue doing such experiments in the future and not interfere with anything.

That's said please vote here:

* Github Actions: https://github.com/actions/runner/issues/2347
* CircleCI: https://ideas.circleci.com/ideas/CCI-I-344 (unfortunately requires a free account to vote)

to get a much simpler support for being able to have a failing step that shouldn't impact the overall PR status.

@LysandreJik, @sgugger 
