Update and remove duplication

The account verification page has really needed a review. Following the completion of gitlab-com/support/support-team-meta#5114 (closed) is a great time to do it.

This MR changes the following:

1. Adds some missing information.
2. Changes heading levels and adds numbering to some headings to make it easier to read through.
3. Updates some inaccuracies/inconsistencies.
4. Moves troubleshooting content to the docs: gitlab-org/gitlab!136395 (merged)
5. Removes duplicate content.
6. Moves Account Ownership Change section to the Account Changes workflow (where it makes more sense to live).
7. Other minor formatting changes

For the confirmation emails workflow:

1. Updates typo fix with guidance based on recent changes