Consolidate documentation about adding database indexes

What does this MR do and why?

Based on the suggestions from @OmarQunsulGitlab in !112241 (comment 1282968033) and #371626 (closed), this MR:

• Moves some information about indexes from migration style guide to database index docs
• Adds links to database review docs that reference database index docs.
• Updates guidance for using a post-migration to be consistent (longer than 10 minutes in Database Lab).

Issue: #371626 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.