Merge branch 'feature/runner-script-attempts-docs' into 'master'

Add new runner script attempts docs and update .gitlab-ci.yml

Related: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1907 and https://gitlab.com/gitlab-com/infrastructure/issues/685

Adds missing docs about the new script attempts variables in the runner.

See merge request !8100