Add custom Sonata page route to the navbar
You should override standard_layout and modify content of side_bar_nav block. This is simple and fast way. Or you can dig into sonata code to find how to inject something into admin_pool.dashboardgroups - have fun :)
I don't think that's possible, you have to create a new layout, copy the sonata admin layout and customize it to your need.
You can change the layout used by changing the yml configuration for sonata_admin (templates -> layout) or extending the SonataAdmin bundle and creating your own layout.html.twig.