Add link to Omnibus downgrade section in the navbar

As stated in https://gitlab.com/gitlab-com/business-technology/engineering/infrastructure/issue-tracker/-/issues/66#note_664270098

The Update/Upgrade documentation does not include a section/sidebar link about downgrades, it is only on the specific deployment method pages below the navigation scrollbar fold so is not easily found.

which resulted in user errors and nuking a GitLab instance. Fortunately, this was a demo.