Add PostgreSQL troubleshooting on measuring (RE)INDEX progress

What does this MR do and why?

This MR introduces information on measuring (RE)INDEX progress to the PostgreSQL troubleshooting docs as a part of the Support Spike on this topic gitlab-com/support/support-team-meta#6377 (closed).

This information should not be needed under most normal circumstances. When the need for this arises, it's important for this information to be readily available.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

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