Document squid proxy usage

### Related software versions

all

### Description

Add documentation on how to use/start the squid proxy, as well as how to plug it in to Surface (this part preferably on [Surface's wiki](https://github.com/surface-security/surface/wiki).

### Use cases

Both as a dev and/or end-user, I want to be able to learn how to use the proxy for my scans.

### References

_No response_