Added the "How Piwik Works" guide

This new guide is an overview on how Piwik works. It's intended for people getting into the codebase (like me).

It covers a bit what's already in other guides but hopefully without duplicating: it's meant to be just an overview. It links to more specific guides when appropriate.

If you see some other major parts of Piwik that would benefit from being in that guide let me know, I can add them.
it is `plugins/` vs `plugin/`