Add new entry to upgrade navigation section

Adds menu item for topic created in: gitlab!64953 (merged).

Also moves some other upgrade-related content.

See:

• New item:
  • Before: https://docs.gitlab.com/ee/update/plan_your_upgrade.html
  • After: http://eread-updates-to-upgrade-navigation-section.178.62.207.141.nip.io/ee/update/plan_your_upgrade.html.
• Omnibus item:
  • Before: https://docs.gitlab.com/omnibus/update/
  • After: http://eread-updates-to-upgrade-navigation-section.178.62.207.141.nip.io/omnibus/update/
• Source item:
  • Before: https://docs.gitlab.com/ee/update/upgrading_from_source.html
  • After: http://eread-updates-to-upgrade-navigation-section.178.62.207.141.nip.io/ee/update/upgrading_from_source.html