Added documentation on adding database indexes

What does this MR do?

This MR adds a guide on when to add database indexes, how to approach this problem, etc

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

Spelling, grammar, etc

Why was this MR needed?

Blindly adding indexes can be harmful to the database, and I recently had to remove 48-something unused indexes.

What are the relevant issue numbers?

#20767 (closed)

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together