Add database documentation on evaluating and dropping unused indexes

What does this MR do and why?

As part of our initiative to reduce the size of CI database tables (CI Tables Are Too Big (&13885)), we opened an issue to remove unused indexes: #498968 (closed). As such, it has become necessary to provide a more detailed guide on how to evaluate indexes and confirm they can be dropped. Since there are risks associated with dropping the wrong index (i.e. one that is still used), this guide could help mitigate potential incidents.

This documentation is based on the Index evaluation guide written in #498968 (closed), which was derived from work done in #520213 (closed).

References

• Cleanup unused indexes in CI tables (#498968 - closed)
• Index Removal: index_ci_job_artifacts_on_expire... (#520213 - closed)

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