[doc] Update "settings" pyramid documentation
Document that without "bwcompat" we can use a development.ini file directly
and add a dedicated section to list configuration options.
[doc] Update the "quickstart" page of pyramid documentation
Again mentioning the two modes of operations and dropping references to prior
implementation.
[doc] Rewrite "pyramid" index page in the book
We drop references to pyramid-cubicweb and cubicweb-pyramid, remaining of the
merge in 3.24. We now mention the two possible operation modes
(with bwcompat being true or false).