Update allowed values of `contributes` in `_extension.yml` docs

[cwickham]

In the _extension.yml docs, the current list of allowed values for contributes is shortcodes, filters, and formats.

I think the missing values are: project, metadata, and revealjs-plugin, (and as of 1.8) brand.

quarto-cli/src/extension/types.ts

Lines 21 to 27 in 7c8fd21

	export type Contributes =
	| "shortcodes"
	| "filters"
	| "formats"
	| "project"
	| "revealjs-plugins"
	| "metadata";