Introduce low-level documentation on Django's components

[Markus Holtermann]

I first had the idea a Django: Under the Hood 2014, but abandoned it. A couple of days ago the idea came up in IRC again:

It would be helpful for new contributors as well as for other core devs not involved in certain parts of Django, to have documentation that outlines how these components work.

I.e. in migrations: you have a ModelState and to detect changes the autodetector takes all migrations and build the final model representation and then compares this ProjectState to the ProjectState constructed from all currently available models.

Tim proposed to add that documentation to docs/internals/. I'd put it into docs/internals/api/