Extend Rancher dashboard navigation documentation

[cnotv]

Description

Define missing point:

• Origin of all the existing types (schemas)
• View where the markup for the sidebar is defined, since we have no component
• Guidelines on how to add a type to the navigation (e.g. new PSA)
• Information regards the navigation configuration mapped over the schemas/types
• Further UI informations, such like:
  • Add link icons
  • Define hierarchy or add children
  • Prioritize sorting

Context

In the current documentation there's several missing info and guidelines for the navigation, specifically side bar.

[gaktive]

This should tie in nicely for a epic for the nav. Need to check with @kwwii when he's back about the other nav tickets and figure out the grouping.