separate recipes page, add tutorial, and examples of extending basic functionality

The title is a bit catch-all... but it's a good time for a small revision of the documentation.

I collected the following list:

• put the recipes section (which may very well grow) on a separate page
• add a tutorial (something longer than on the install page, could maybe go on the 'userguide' one? I could reuse the material from last Friday)
• add some examples of extending the basic functionality (loading a C++ module, taking additional command-line arguments from a module). Should this be another 2 recipes, or become and "advanced topics" or "under the hood" page (together with "ordering cuts and selections, perhaps?)

Opinions / suggestions (within the same scope)?