[doc] more fixes of warnings/errors in doc build - move 3.21.rst changelog description at it's proper place - include changelogs in the main index - few typos - add :noindex: for autoxxx directives not in api/ Related to #4832808

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl