Replace template with quarto

[dgkf]

Replaces built-in template with the standalone quarto template.

This is effectively a total rewrite of the package. Just to outline where all the functionality went:

• special elements (tables, cards, etc) now handled by the quarto template, so they should be more portable, be able to make use of _brand.yml stylings and be easier to extend for other formats.
• riskmetric metrics replaced with val.meter metrics
• license cards are not yet implemented, but the goal is that it moves into the quarto template.

---

still to do...

• add more useful message when trying to work with inst that does not have submodule cloned
• update tests
• update README (definitely include instructions for cloning with submodules)
• update documentation
• allow custom _brand.yml or brand list()

[dgkf]

Since we've discussed this change at a few weekly meetings and the overall change should be uncontroversial, I'm just going to merge in and we can fine-tune incrementally from there.