Consolidate file-related resources together

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!155608 (merged)
  • Old location: doc/user/project/repository/file_finder.md
  • New location: doc/user/project/repository/files/index.md
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Related items

• Related to Rename page from file_finder.md to files/index (gitlab!155608 - merged) where I moved a page
• Related to https://gitlab.com/gitlab-org/gitlab/-/issues/457784+ which tracks the overarching work