Documentation + rubocop for checking that foreign key constraints require an index

What does this MR do?

Adds documentation and a rubocop. See https://gitlab.com/gitlab-org/gitlab-ce/issues/49789.

The rubocop does not match the case where a column exists already and we add the FK constraint with add_concurrent_foreign_key. This is because we might already have an index on that column and I do not see a way to check for this. Is there?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/49789