Pajamas documentation updates based on navigation redesign

Purpose

With the coming changes for the navigation redesign (gitlab-org&9044), there are a variety of documentation updates that we will need to include for Navigation.

Considerations

• Review conflicting areas of documentation between handbook and Pajamas
• Use of the chevron icon with menus (#1472)
• Guidelines for navigation groupings (gitlab-org/gitlab#378545 (comment 1208243977))
• How can different sections of the sidebar look? When in settings, a user profile, or in search, admin area, groups / projects, explore, dashboards, etc.
• What belongs in the left sidebar?
• Which areas get a dedicated left sidebar?
• Should it be called a sidebar? Panel maybe?
• What determines which items belong in top top static section of the contextual sidebar area? What should be the limit here?