Allow selecting the org.gnome.Sdk.Docs runtime extension
We currently load all documentation found in the Flatpak run time used by Devhelp, which works well enough for viewing the documentation of the current release of GNOME.
Ideally, though, we want to be able to list all the available run times with the org.gnome.Sdk.Docs
extension installed; once selected, we need to instantiate the sandbox for the run time, and navigate in that sandbox.