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 :-)
I've added it here because in custom-lookups we have something like

> We then need to define the ``as_sql`` method. This takes a ``SQLCompiler``
object, called ``qn``,  and the active database connection. ``SQLCompiler``
objects are not documented,  but the only thing we need to know about them is
that they have a ``compile()`` method which returns a tuple containing a SQL
string, and the parameters to be interpolated into that string.