[discussion] Create docs to explain difference with Radix?

## Summary

A lot of people are asking us the same question, over and over again. What's the difference between Radix Primitives and Base UI? Examples:

- https://x.com/conacts1/status/1979171731316064681
- https://x.com/firatoezcan/status/1974408959781187935
- React Conf attendees question https://www.notion.so/mui-org/2025-React-Conf-Trip-report-1d6cbfe7b66080309349c0224fb148f9
- The main non component Google search is also about this: https://search.google.com/search-console/performance/insights/queries?resource_id=sc-domain%3Abase-ui.com&time_range=LAST28DAYS&queries_tab=TOP.

## Examples in other libraries

Some libraries have comparison pages, to help understand the difference.

## Motivation

Clarity. Even if we don't act on this, simply having a GitHub issue (here), that people can find on Google is already a win.

It also relates to #2970.

## How

This could be executed with a docs page.