[docs] Migrate Pigment CSS docs to pigment-css.mui.com

[oliviertassinari]

Steps to reproduce

Search for "Pigment CSS" on Google. Get a first result in https://github.com/mui/material-ui/tree/master/packages/pigment-css-react. Most importantly, this repository doesn't surface, so good luck finding it to leave a star 🙃.

This is all wrong:

• 1. This page shouldn't exist in the first place. update the content to only link to the new location. [pigment-css][docs] Fix duplication of content material-ui#42410
• 2. Delete https://github.com/mui/material-ui/tree/master/packages/pigment-css-react
• 3. We should have https://pigment-css.mui.com live
• 4. We should ask Josh to update its link in https://www.joshwcomeau.com/react/css-in-rsc/#pigment-css-9 and in https://www.joshwcomeau.com/talk-resources/react-paris-talk-2024/ to not have broken links.