Add missing install/update topics to left nav

Add navigation item to global navigation

The following pages are missing from the left nav:

• https://docs.gitlab.com/ee/update/mysql_to_postgresql.html -> https://kpaizee-main-patch-82677.docs.gitlab-review.app/ee/update/mysql_to_postgresql.html
• https://docs.gitlab.com/ee/update/restore_after_failure.html -> https://kpaizee-main-patch-82677.docs.gitlab-review.app/ee/update/restore_after_failure.html
• https://docs.gitlab.com/ee/update/upgrading_postgresql_using_slony.html -> https://kpaizee-main-patch-82677.docs.gitlab-review.app/ee/update/upgrading_postgresql_using_slony.html

As identified in #1313 (closed) and related to technical-writing#712 (closed).

These pages are listed here https://docs.gitlab.com/ee/update/#miscellaneous, so they don't appear to have been excluded on purpose.