docs: clarify some points in the guides for first-time setup

[thegamecracks]

Makes a few adjustments to the documentation based on issues I had setting up OCAP for the first time:

1. Setting prefixURL and serverUrl to match accordingly
2. Setting OCAP_AUTH_ADMINSTEAMIDS env var to a comma-separated list of IDs
3. Needing to (not rather, not needing to) upload map tiles to the OCAP webserver
4. The difference between uploading map tiles from Google Drive / archives.ocap2.com versus using ghcr.io/ocap2/web:full to process grad_meh exports into map tiles

Hopefully this saves a bit of headache for the next person setting up OCAP :)

[thegamecracks]

Awaiting answers to these questions so I can make amendments for the map parts:
https://canary.discord.com/channels/858402673066115102/871034165271855125/1532814945077559407

1. Is it necessary or otherwise preferred to host maps locally? That got me confused while reading through the readme and guides, as I was assuming I had to download the maps that I planned on recording when it turned out the panel could fallback to CDNs. Whatever the explanation, I'd like to add it to the guides' corresponding sections.

4. Serving local maps from google drive (or archives.ocap2.com) doesn't require the :full Docker image, right? I'd imagine they're pre-processed after cross referencing a bunch of documentation, but seeing the install guide describe a "map tile processing pipeline" and not knowing grad_meh exports vs. rendered map tiles beforehand, I want to slightly clarify the wording that it's only for processing those exports.