plotly.css file should be added to the release files

[inari-mchristie]

          > Note: The `dist/plotly.css` file will need to be added to the release files.

I don't see any plotly.css file anywhere. Is this expected? Should it be added to the dist/ directory?

This feature should also be documented somewhere. Right now I think this PR is the only way to learn about it.

Originally posted by @maryla-uc in #7109 (comment)

[inari-mchristie]

I was able to build the plotly.css file manually but it would be better if it was added to a distribution so that folks can take advantage of the CSP work done in #7109

git clone --branch v3.0.0-rc.2 https://github.com/plotly/plotly.js.git
cd plotly.js/
npm i
npm run preprocess