Add form-associated components documentation.

[nicholasrice]

We should add documentation to address questions about our form-associated custom elements. We should address:

1. What is form association? What does it enable?
2. Which components are form-associated?
3. Approach to down-level browser support.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[usrrname]

Here yuh go: https://stackblitz.com/edit/vitejs-vite-154zcd?file=index.html

https://stackblitz.com/edit/vitejs-vite-154zcd?file=README.md