Fix two Omnibus links in the left nav

As part of cleaning up the docs redirects for September 2021, some Omnibus links turned up (hi @axil) that are slated for deletion but still appear in the left nav. Let's get those fixed before we start deleting redirects and outdated redirect files.

• Link to the merge request of the new page: n/a

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.

Related info

• Related to technical-writing#474 (closed) where I'm doing the monthly chores
• Related to omnibus-gitlab!5607 (merged) where the old Omnibus pages are being removed
• Related to !2135 (merged) where the redirect entries are slated for deletion
• Related to gitlab!70423 (merged) where gitlab redirects are being cleaned