Document out-of-repo App Engine configuration

[gsnedders]

Currently, we have in our docs:

https://github.com/web-platform-tests/wpt.fyi/blob/71fd8da3e1bb7ae76dc05efa63e20d920858fb4b/docs/app-engine.md?plain=1#L51-L56

This isn't particularly useful — even if we don't use Terraform or similar IaC tools, it would be good to document what they are.