Clarify facility, site, and resource group naming

From https://support.opensciencegrid.org/public/tickets/2f0465dda314583de2cf9399a9e4e1dbff39ee7ea222258147c690fdd7a7d1b2:

Points in the documentation that I found confusing:

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2) The instructions use the terms "<FACILITY>", "<SITE>", and "<RESOURCE
GROUP>". It was not clear to me what to use for those.

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1. Wherever we use <FACILITY>, <SITE>, or <RESOURCE GROUP>, let's link back here: https://opensciencegrid.org/docs/common/registration/#registering-resources

2. Let's pick a naming scheme for resources and document it in the table. Something like <VO><COUNTRY CODE><ABBREVIATED SITE>_<RESOURCE>, e.g. GLOW_US_CHTC_SQUID2. If they don't know their VO, they can just use OSG.