[docs] Create "Accessibility" h2 / guideline for each component?

## Summary

The docs of Base UI could be holding my hand on how to make the component accessible. What if we were to create more content around this? This feels like there is depth for an a11y engineer to do technical writing work here.

## Examples in other libraries

- https://www.radix-ui.com/primitives/docs/components/toast#accessibility
- https://mui.com/material-ui/react-snackbar/#accessibility

## Motivation

Clear guidelines, avoid people from getting confused, e.g., https://bsky.app/profile/therealkimblim.bsky.social/post/3lyuiuaodmc27.