[Sync to prerelease] unified brand, dark logo, etc

[github-actions]

Description

Sync of #1779 to prerelease.

Original PR

unified brand, dark logo, etc

@cwickham

This is not 100% complete so feel to wait on reviewing this. I'll put as draft until I've given it another pass and reviewed PRs to figure out what I've missed.

However, it does include all the big changes

• unified brand (with light and dark colors) is preferred over light and dark brands
• brand-mode
• removed some limitations and added others (but they are smaller)
• document logo can have light/dark variants (and also mention overriding with path)
• typst logo special options moved to the main typst page (introduced with brand, but can be used without it)

So I thought it was worth opening a PR now!

I feel like the new page.typ partial should be documented somewhere, but we do not copy or document some of the other Typst partials, so I'm not sure if it belongs here:

https://quarto.org/docs/output-formats/typst-custom.html

The docs list 2 but there are now 7:

biblio.typ		page.typ		typst-template.typ
definitions.typ		template.typ
notes.typ		typst-show.typ

[cderv]

/deploy-preview