[doc] get back baseviews documentation into the code, enhance it and reorganize the module accordingly
also properly deprecates SecondaryView as it should have been done a while ago.
Startup views
-------------
(:mod:`cubicweb.web.views.startup`)
The usual selectors are no_rset or yes. These views don't apply to a
result set.
*index*
This view defines the home page of your application. It does not require
a result set to apply to.
*schema*
A view dedicated to the display of the schema of the instance