Miscellaneous demo ideas

Thanks for making this. I wanted to dump some ideas for other treed demos that I think could be interesting. I'd like to get around to writing these. I just wanted to document them here and also gauge your interest.

- `org-mode` for Emacs users
- markup for HTML or React components
- json, yaml, and other config formats

No source files, tests, or entry points are named. Start by reviewing the existing demo structure, then determine which proposed format has maintainer interest; done means selecting a specific demo and defining its implementation scope.

Written by the indexing model from the issue text.