Docs: Update redirected links in Omnibus docs

See the general Documentation guidelines: https://docs.gitlab.com/ee/development/documentation/.

What does this MR do?

The docs site generation identified some links that are being redirected. This MR updates all of those links.

It also fixes heading nesting in two readmes, and cleans up two markdown tables, since I was there already.

Related to https://gitlab.com/gitlab-com/gitlab-docs/issues/394