Improve installation instructions with pg_ctl in documentation

Improve installation instructions with pg_ctl in documentation

The documentation includes sections to be able to initialize and start
Postgres via a couple of commands.  Some of its recommendations involve
using directly "postgres", which is inconsistent with the recommendation
given by initdb.  At the same time make some other command calls more
consistent with the rest, by using an absolute path when creating a
database.

Author: Andreas Scherbaum
Reviewed-by: Michael Banck, Ryan Lambert