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.
what i mean is that `log_action` is a table used as a hash table that stores all the unique Page URLs, page titles, download Urls, outlinks urls, event names, event actions, etc.
then in the table log_link_visit_action we simply use the Action IDs from this lookup table which contains (once) each Action found on the websites.