Add reference to product map.

What does this MR do?

Mention the product map in the UI Guide and remove a line that says that the GitLab logo and user picture and in the sidebar.

What are the relevant issue numbers?

19350

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together Closes #19350 (closed)