document our fastly/CDN setup

so we have a CDN we use here, and it's not really documented. we have fairly good docs on the static-component system, but nothing on Fastly. we didn't even have a tag for it until #40978 (closed) was filed (and i made it).

so we should document:

• what we use fastly for
• how it's configured (e.g. cdn-config-fastly.git, ./tor-puppet/modules/roles/files/puppetmaster/update-fastly-ips, static-component yaml file, probably more)
• what talks to it and why not everything is on there
• what our limits are
• contact information
• password management

basically make a full service audit.