Add a new section on migration type to use
What does this MR do and why?
During a review, it's come up that https://docs.gitlab.com/ee/development/database/adding_database_indexes.html did not mention/link to the migration style guide that explained which migration type to choose.
This documentation update adds a link to the migration style guide and explains some common scenarios encountered when choosing a migration type to create an index.
Edited by euko