Make it easier to update the navigation bar of the user documentation

Currently , the navigation bar, especially the burger menu, is hard-coded in every single html page for the user documentation (in rcp.help). This makes it difficult to change/add new sections (in the burger menu), because then every other html page must be updated as well.

It would be better if the navigation bar would be editable in a single place and then would be automatically updated in all other pages (something like imports?).