[book] Improve PostgreSQL configuration section
* Do not advice for specific PostgreSQL version (point to meta-packages if
available in Linux distros);
* First mention packages from Linux distros, put the link to official postgres
documentation at the end;
Closes #2725302.
#!/bin/sh -e
if [ "$1" = "purge" ] ; then
update-rc.d cubicweb remove >/dev/null
fi
#DEBHELPER#
exit 0