Doc: mention packager-supplied tools for server start/stop, initdb, etc.

Doc: mention packager-supplied tools for server start/stop, initdb, etc.

The majority of our audience is probably using a pre-packaged Postgres
build rather than raw sources.  For them, much of runtime.sgml is not
too relevant, and they should be reading the packager's docs instead.
Add some notes pointing that way in appropriate places.

Text by me; thanks to Daniel Gustafsson for review and discussion,
and to Laurenz Albe for an earlier version.

Discussion: https://postgr.es/m/159430831443.16535.11360317280100947016@wrigleys.postgresql.org