doc: add a cookbook

[emillon]

This creates a "cookbook" section with some self-contained examples.

Theses examples are extracted from quick-start.rst.

One nice aspect is that these samples are linkable, and that the previous/next links work.

I tried to convert some of the examples to a style that encourages tinkering ("Run it!" / "Change it!" in hello-world for example), but most examples are just moved as is.

Questions / follow-up work:

• in terms of diataxis, this is tutorial material. should this leave under /tutorials/cookbook?
• the "building a hello world program from scratch" is the closest we have to a tutorial. it could be recycled into a proper one.
• also, this tutorial is the only place we explain that the various files need to be created with a text editor and put in the same directory. this could be added to cookbook/index.
• some recipes are not self-contained (ex: define-library), are not formatted (ex: generator), do not follow best practices (ex: c-stubs, useless (name), no :standard), or might be useless (toplevel). This can be addressed separately.
• what about our examples directory? should it be moved to the cookbook?

[emillon]

Curious to hear your thoughts about this @ElectreAAS