[DOC] Documentation on data-request-flow

I would like to know more about how the information flows inside and outside nitrogen:

• inside nitrogen: which module calls which one to do what
• outside nitrogen:
  • what kind of requests are there (Page-Rendering, Comet, etc.)
  • how are these submitted (POST, GET, including especially their parameters, name of the parameters, etc.)

so we can ultimately draw some nice "information flow diagrams".

These diagrams might become quite usefull for the other open tasks like the problem wrt make_ref(), how encoded and what we store in cookies, what gets serialized in the java scripts, etc.