Update global navigation for files renamed from README.md to index.md

Part of: #864 (closed).

The README.md files were renamed to index.md, but the menu needs to point to the new files, else it default closes. Example:

• Before: https://docs.gitlab.com/omnibus/development/
• After: http://eread-update-nav-for-renamed-files.178.62.207.141.nip.io/omnibus/development/