[docs] Add "Supported platforms" page

[oliviertassinari]

Summary

Have a page that can be the source of truth for what browsers and other third-party tools support downstream projects should use.

Motivation

We have people asking questions like mui/mui-x#4973. Today, we tell them that they should refer to https://mui.com/material-ui/getting-started/supported-platforms/ but since Material UI is no longer the most upstream library, now Base UI / Pigment CSS are, we need to set a baseline here.

[colmtuite]

MUI, Joy UI, and other libs built on top of Base UI, should communicate their own requirements. A styled lib built on Base UI might rely on React 19 for styling, or might use text-box or some other CSS property with limited support. In all cases, Base UI supported platforms will be a superset of the lib's supported platforms, so it's not useful to link to Base UI for this.