Render index.* like README.* when it's present in a directory

What does this MR do?

Adds an alternative name for README file: /index[^\/]*/.

From https://gitlab.com/gitlab-org/gitlab-ce/issues/18933:

If we could render index.md like README.md then we would have a 2-way profit:

• When visiting https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/api the page would still render the contents of index.md

• Static generators would create easily index.html files from the relevant index.md

The main point is clean URLs.

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

I’m not sure.

Why was this MR needed?

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [ ] Internationalization required/considered

What are the relevant issue numbers?

#18933 (closed)