[doc/book] Add a note about <database>-support virtual package. Cubicweb setup documentation now mention cubicweb-postgresql-support and cubicweb-mysql-support

bgcolor = '#000000'
stylesheets = ['%s/cubicweb.css' % datadir_url]
logo = '%s/logo.png' % datadir_url
lazy = lazystr('%(bgcolor)s')