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

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

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.

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

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