[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.
The 'download' views
====================
.. automodule:: cubicweb.web.views.idownloadable
Components
----------
.. autoclass:: cubicweb.web.views.idownloadable.DownloadBox
Download views
--------------
.. autoclass:: cubicweb.web.views.idownloadable.DownloadView
.. autoclass:: cubicweb.web.views.idownloadable.DownloadLinkView
.. autoclass:: cubicweb.web.views.idownloadable.IDownloadablePrimaryView
.. autoclass:: cubicweb.web.views.idownloadable.IDownloadableLineView
Embedded views
--------------
.. autoclass:: cubicweb.web.views.idownloadable.ImageView
.. autoclass:: cubicweb.web.views.idownloadable.EHTMLView