Allow custom CSS to be loaded from assets

[pvande]

Proposal: Add support for a new frontmatter property that can refer to a CSS asset file in the documentation, to be linked or injected into the generated content.

Rationale: Custom Components aren't able to take advantage of Tailwind classes. Some projects may want to manipulate the presentational style of built-in components.

Current Workarounds: At present, it is possible to implement custom CSS using a hack like the following:

<style children="dl.not-prose { display: grid; grid: auto-flow / max-content 1fr; gap: 0.8em }" />

While this is functional, it adds redundancy and overhead to the MDX source files when it's copied across many source files.