GitLab Next

- Moves /source/handbook/engineering, /source/handbook/marketing, and /source/handbook/index.html.md
  under /sites/handbook/source/handbook
- See #6689

They need to be ERb templates if they're using Ruby syntax.

This commit replaces "sub-department" with "section".  Since i used
MacOS sed, it also adds newlines to end of files.  File extensions
touched were .erb, .md, and .yml.  Exceptions were the changelog,
rss feeds and  announcements page.