Restructure expressions documentation to better highlight ORM support

[Josh Smeaton]

The docs in docs/ref/models/expressions.txt could probably do with an overhaul. The original document was very specific about highlighting the use-cases of F expressions in terms of filter and update.

I think the docs should be structured in terms of which queryset methods support expressions. We can then leave the specific expression reference below.

Some of the F() examples can be pulled out into the queryset method sections, but the F() section can still highlight the things it is particularly good at -- like avoiding race conditions.

The docs would be structured like:

=================
Query Expressions
=================

Intro..

Supported QuerySet Operations
=========================

Not every :doc:`QuerySet </ref/models/queryset>` method has full support for
Query Expressions, but the support is growing with each release. Below is a
quick overview showing which methods do have support.

create() and update()
----------------------------------

Intro

Examples

filter() and exclude()
--------------------------------

Intro

Examples

[Mariusz Felisiak]

The current version of ​expressions reference contains reference material for many expressions and it seems to follow the principles outlined in the ​​Diátaxis framework. Closing as "invalid".

Other improvements related with expressions docs are tracked in #25703.