Add files from development/backend

• Link to the merge request of the new page:

  • gitlab!68538 (merged) added the index page
  • gitlab!85539 (merged) added more pages to the directory
  • No idea when the Ruby style guide page was added; I just spotted it on the way

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.

Related links

Related to https://gitlab.com/gitlab-com/create-stage/create-engineering-managers/-/issues/416 which is the source of Sean's work.