Document examples of server URLs

The docs only mention datafind.gw-openscience.org as a GWDataFind host. Users running on LVK computing clusters will not need to specify a host, as it's set by default (e.g., ldr.ldas.cit on CIT machines). I've Outside of that, there is no mention of available hosts, at least not that I can easily find. None of these can be used to access non-GWOSC data from an external machine, but the undocumented (as far as I can tell) datafind.ligo.org handles precisely this. I propose keeping a table in the Sphinx docs listing all of these hosts, where they're accessible from, and what data they have access to. Something like:

Hostname	Accessible by	Data available
datafind.gw-openscience.org	Anybody	GWOSC public data
datafind.ligo.org	authenticated LVK members	All data
ldr.ldas.cit	CIT cluster users	All data
...	...	...