[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
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/