Show GeoServer version more prominently on the doc pages

Currently, the only ways to know which GeoServer version a documenatation page is referring to is by looking at the Title bar or by parsing the URL. I think it would be more helpful to see the GeoServer version prominently at the top of the page (as part of the theme) Optimally, it would be above the breadcrumbs (Home >> Foo >> Bar), but if that's not possible, then immediately below it.

The string to use would be the "release" variable, as seen in the conf.py config file.