[docs][umbrella] Create Pigment CSS docs at pigment-css.newco.com or pigment-css.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 🙃.

Improvements

• 1. https://github.com/mui/material-ui/tree/master/packages/pigment-css-react 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. Move as much as possible content from https://mui.com/material-ui/experimental-api/pigment-css/ to https://github.com/mui/pigment-css/blob/master/README.md. Use https://mui.com/material-ui/experimental-api/pigment-css/ to host docs that is Material UI specific on how to use Pigment CSS. I would dream of a world where this page is useless, as Shadcn UI has nothing about how to use Tailwind CSS, it's naturally extended it. But still, I gues we will need content there, we are not there yet.
• 4. https://mui.com/material-ui/migration/migrating-to-pigment-css/ is confusing. It is located in a place that makes it seem like it's about Material UI v5 to Material UI v6. But it's about Emotion to Pigment CSS. Move the content to:
  • In https://github.com/mui/pigment-css/blob/master/README.md if the production direction is to have: Material UI = Base UI + Pigment CSS, nothing in between, or to have Base UI + Pigment CSS System, nothing in between. Pigment CSS has an emotion runtime engine + a static extraction engine.
  • In https://mui.com/system/getting-started/ if the production direction is to have Material UI = Base UI + MUI System (wrapping Emotion and Pigment CSS).
• 5. We should have https://pigment-css.mui.com live
• 6. 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.
• 7. Create Why page [docs] Create Why Pigment CSS page #210
• 8. Use videos through the docs, we have two so far that we don't use enough: https://www.youtube.com/watch?v=UVeDpUey5Es and https://www.youtube.com/watch?v=n_0cz-JwlsU.