Update omnibus-nav.yaml with missing docs pages

Similar to what was discovered in gitlab#201828 (closed), the Omnibus docs don't appear to have all of the pages included in the left-hand navigation.

I grepped for one of the pages included in the left-hand navigation, and limited the result to YAML files, which pointed me to:

$ grep -rl "common_installation_problems/README" . | grep yaml

./content/_data/omnibus-nav.yaml

Review the instructions at https://docs.gitlab.com/ee/development/documentation/site_architecture/global_nav.html and add topics to the global navigation.