Fixed #22812 - Reformat and restructure lookup API documentation - NOT RFC

Ticket [#22812](https://code.djangoproject.com/ticket/22812)

The commits are separated to facilitate reviewing: all text related can be reviewed on commit 12acee0 and 41a4494. The middle commit is just renaming with cascade to other docs.

- 12acee0 adds the references for `RegisterLookupMixin` and correctly formats attributes and methods of class `Transform` and `Lookup`, also improving some text (not worrying about 80 lines wrap and such).

- 41a4494 focus on improving the ref/models/lookups.

For now, I would like to ask for a general feedback on the structure and format, in particular to @akaariai: there may be things that are not correct.

There maybe parts of refs/models/query.txt that could be here, but I will bring that discussion to the ticket.

Once we have a structure defined, I will start working out 80 chars wrapping, fixing documentation warnings, etc to make it RFC :-)
The "first element is always the name of the class that registered the lookup" isn't correct. The query expression (or maybe "lookup expression"?) consist of three parts:
  1. Fields part (Book.objects.filter(author__best_friends__first_name...)
  2. Transforms part (may be omitted) (__lower__first3chars__reversed)
  3. A lookup (__icontains). If omitted, Django uses exact

In addition "name of the class" isn't technically correct.