Document truncate_legacy_tables rake task

What does this MR do and why?

In !94252 (merged), we introduced rake tasks for truncating main data from ci database and ci data from main database.

This MR will document these rake tasks. The documentation will be added to https://docs.gitlab.com/ee/development/database/multiple_databases.html

Related to #401683 (closed)

How to set up and validate locally

• Tested locally using instructions found on https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md

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.