[myst] Implement support for custom CSS with myst-theme

[agoose77]

Jupyter Book 1 supported easy injection of custom CSS and JavaScript into webpages. Jupyter Book 2, via the MyST Engine, does not support this. This user-story tracks the need for that feature.

Suggested Actions

1. Add support to MyST's site frontmatter on a page/project basis to declare custom CSS bundles
2. Update mystmd and myst-theme to include and consume the artifacts
3. Consider needs for remote CSS (url), SCSS, tailwind, etc.

As part of our "tactical upstream work", this issue tracks the implementation of this feature.

See upstream issue jupyter-book/myst-theme#321 and upstream PRs jupyter-book/myst-theme#540 & jupyter-book/mystmd#1873

Definition of Done

• PRs adding support for hand-authored .css files added to mystmd and myst-theme
• Documentation for custom CSS is included in that PR(s).

[agoose77]

I've conservatively given this an 8, because I hope it will be more like a 5, but this work involve touching parts of the stack that are not regularly tweaked and may be fragile.

[agoose77]

I've revised this to a 2, because I don't think we actually need to implement Tailwind support in myst-theme for this!

[agoose77]

This is now done. Opening a tracking issue to follow up.

[agoose77]

Actually, that's not our policy. This lives in "In review/blocked" until sprint-end, at which point we migrate.