make (retroactive) proposal for DoS subsystem

In #24902 (moved), dgoulet speaks of a ddos-design.txt document.

But there is no actual proposal for the overall DoS subsystem.

If we have the document around, and we just never published it, this is a great chance to notice, clean it up a bit, and call it proposal three-hundred-and-something. (And then maybe turn some of it into one of the spec files if that makes sense, but, one step at a time here. :)

Motivated by this month's tor-dev thread where all we have to show for the DoS subsystem design is a trac ticket number and a changelog entry.