Consolidate newlines guide to Ruby guide

What does this MR do?

Consolidate newlines guide to Ruby guide.

This newlines guides is not as well known. Hopefully, this makes it more discoverable.

Also the newlines guide is entirely Ruby specific, no need for another special page.

Related issues

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/index.html#move-or-rename-a-page

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• [-] Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• [-] Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• [-] Update the link in features.yml (if applicable).
• Assign one of the technical writers for review.