Draft: migrate Architecture docs

[f18m]

Problem: ZMQ internal architecture is documented only (AFAIK) in the old website http://zeromq.wikidot.com/whitepapers:architecture

Solution: bring libzmq-specific architecture docs into libzmq github repository

[f18m]

Hi @bluca ,
let me know what you think about this PR (not completed yet).
I think the page http://zeromq.wikidot.com/whitepapers:architecture was really useful to me when I first started debugging libzmq internals.

Given that the old wiki is sort of abandoned, I think this valuable information should find a new home. A possibility is, as I did here, to integrate the architecture definition in official docs.
Other possibilities would be:

• enable the GitHub wiki feature and re-create the architecture page there.
• add the architecture page as a MarkDown/AsciiDoc page under github but exclude that from the manpages and from the HTML docs published by readthedocs.io (rationale: this is internal docs for advanced users only)

[bluca]

This is fine by me, just one thing, can you use svg for the images? I think it works better in git for storage purposes