migrations: Update `dirty database` docs

Created by: efritz

This PR is a rough docs update on how to handle a dirty database. This PR adds a new commands sg migration add-log, and migrator add-log, which can be used by developers or site-admins to help resolve the dirty database conditions with the new migration infrastructure.

Reviewers: Please give special attention to the doc page and suggest where I can steal copy from other migrations work we've been doing lately.

Edit: Non-docs work was pulled out into https://github.com/sourcegraph/sourcegraph/pull/30863.