Improve documentation for proxying the scripts

[tazz4843]

Right now, the current documentation (https://docs.simpleanalytics.com/proxy) is pretty good, but I ran into some issues and confusion while trying to set it up myself (I did eventually get it in the end though!).

Couple use-cases for this I can think of if you're curious:

1. uBlock Origin on Firefox blocks CNAMEs that link to a blocked site
2. If all the traffic originates from the same server, it can boost performance and allow slightly more advanced caching
3. It'd be easier to put it behind something like Cloudflare and load faster

I didn't really want to have to go through and manually change each page to proxy.js, so docs for proxying just the script and the pixel through would be awesome. I can provide my own example of my NGINX config if needed for that.

I also wanted to host it on a different domain than my site itself, which the existing solution didn't exactly go into.

Thanks!