Docs: Demo Setup

[mihar-22]

Base UI uses a createDemo utility which uses abstractCreateDemo to create visual demos like this code for this page. It supports multiple variants/demos. They also use the same demos in their E2E testing.

If we can get it to support multiple frameworks as well then it would enable:

1. Development: Interactive sandbox for building components in isolation.
2. Documentation: Live examples embedded in docs site.
3. Testing: Same demos used for E2E/visual regression tests.

Blocked by:

• UI: Play Button Component #265