Make advanced ORM functionality more discoverable

[Lily]

At Django on the Med we were discussing how many Django users reach for raw SQL because they don't realise that Django provides tools to allow building their query with the ORM.

I think there are several things we can do to improve the documentation here:

• Add a how-to page explaining how to create custom database functions with Func, custom lookups with Lookup and custom transforms with Transform.
• Refactor the lookups reference page (​https://docs.djangoproject.com/en/5.2/ref/models/lookups/) to include a list of the builtin lookups or create a new page.
• Add a page listing the builtin transforms.
• Add crosslinks between the how-to page and the reference docs for lists of builtin functions, lookups and transforms.

[Lily]

For the how-to page, I was considering using ​`LastDayOfMonth` as a worked example.