Finish improvements to the CI configuration for documentation changes (#9460)

* Add bootstrap postjob to CI config

Add a new job to the bootstrap.yml GitHub action config.
This job succeeds if, and only if, all the other bootstrap
jobs succeed.

* Do not run bootstrap CI jobs for documentation changes

The approach was already introduced in #9355 for the validate jobs.
This commit introduces the same change also for the bootstrap jobs.

* Also ignore CONTRIBUTING.md and README.md in CI

We do not run the entire CI suite for documentation changes.
Previously, only changes which were restricted to the 'docs/'
subdirectory were considered to be documentation changes.
With this commit we also recognize changes to README.md and
CONTRIBUTING.md as documentation changes.

* Document improved CI for documentation in CONTRIBUTING.md

The CONTRIBUTING.md file now mentions that documentation changes
do not waste expensive CI resources.

* Recognize all README.md in subdirs as documentation

Expensive CI jobs should not run on changes which affect only
README.md files.