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.
maybe I'm misunderstanding the purpose of this table but regarding the wording:

- to me "action" (or "unique action") means an action that was performed on the site, like "visitor X visited page Y on date Z"
- "action type" means to me the kind of action that can be performed, e.g. "visiting a page", "downloading a file", …

So that's why for the next line I have put:

> - `log_link_visit_action` contains one entry per action of a visitor (page view, …)

-> these are the actions performed on the website

Maybe by "unique action" you mean something related to unique visits?