Remove option to disable Gitaly in Rails

What does this MR do?

• fix oversight in Gitaly token authentication documentation
• link to Gitaly admin documentation from doc/README.md
• remove the option to disable Gitaly completely

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Gitaly can't remain optional forever.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Part of gitaly#346 (closed)