Add doc/dev/Style.md with an imprecation against "onion" to mean HS

This keeps happening. This may be common usage in some parts of the Tor Project, internally, but it is super confusing and I think it should be avoided.

Text stolen from the description in #756 (closed).

CC @ahf since I fear this will be controversial and @nickm since he's familiar with context from before

I wondered if I could somehow make something that would catch this in CI. I ended up with git-grep -iP '(?<!\.)(?<!host::)onion(?!(?:\s*|-|_)(?:service|addr|suffix|key))' and it seems the use I am trying to deprecate is common in local variables at least. I think that's probably undesirable but I don't feel like trying to map the territory and drain the parts that are swamp.