Draft: Add an option to install gitlab-docs service with Omnibus

What does this MR do?

Contributes to gitlab#214164 (closed)

Problem

We want to ship documentation pages in the Omnibus.

Solution

1. Fetch gitlab-docs project
2. Create a symlink to documentation pages from gitlab-rails in gitlab-docs
3. Run bundle exec nanoc to compile documentation files
4. Compress the result with gzip and remove originals
5. Serve gzipped versions of files via Nginx

Later improvements

Move docs folder to gitlab-docs instead of creating a symlink to it. It will reduce the total size of the package.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened

Closes #6552