[pkg] various fixes
- add missing files to MANIFEST
- exclude doc/changes from dh_installchangelogs (it's a directory, which
dh_installchangelogs doesn't expect)
- fix doc build in debian package
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/_build/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc