Merge branch 'document-when-to-add-indexes' into 'master'
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 ## Does this MR meet the acceptance criteria? - [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md) - Tests - [x] All builds are passing - [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides) - [x] Branch has no merge conflicts with `master` (if you do - rebase it please) - [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits) See merge request !5771
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