Add redirects raketask

This will replace the individual raketasks that were created:

• GitLab gitlab!62131 (merged)
• Omnibus omnibus-gitlab!5275 (merged)
• Runner gitlab-runner!2923 (closed)
• Charts gitlab-org/charts/gitlab!2018 (closed)

Nice to haves:

• Add the redirects to redirects.yaml directly.
• Automatically create MRs for all projects.

Related issues

technical-writing#439

How to test

The raketask automatically creates the branches and merge requests (if any redirects are found), The caveat is that if more than one person runs this on the same day, the merge request creation will fail since the branch name will be the same.

bundle exec rake docs:clean_redirects