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

What does this MR do?

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

Replaces Community contribution https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16550

What are the relevant issue numbers?

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

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~

Further checks

• Test that all possible variations without an extension (plain-text) render correctly (test project https://gitlab.com/axil/readme)