[core] Add cross-env to docs:size-why

[merceyz]

• I have followed (at least) the PR section of the contributing guide.

• Added cross-env to the docs:size-why command.

Was trying to find the size of the compiled docs website, when yarn docs:size-why failed due to a missing cross-env.
However I was hoping for an output similar to the one from yarn workspace @material-ui/core build, is there a possibility to add that to yarn docs:build as well?

[mui-pr-bot]

No bundle size changes comparing 35d528c...cf01a9a

Generated by 🚫 dangerJS against cf01a9a

[eps1lon]

$ DOCS_STATS_ENABLED=true yarn docs:build or $ yarn cross-env DOCS_STATS_ENABLED=true docs:build if your shell doesn't support the first syntax.

$ yarn size:why is probably not that usefull. I recommend using yarn size:snapshot and using ./scripts/sizeSnapshot/build/stats.json for analysis. webpack-analyse, webpack-bundle-analyzer or webpack-visualizer are good tools.

[merceyz]

$ DOCS_STATS_ENABLED=true yarn docs:build or $ yarn cross-env DOCS_STATS_ENABLED=true docs:build if your shell doesn't support the first syntax.

That's what yarn docs:size-why does, however it keeps failing due to the JS heap running out of memory.

I wanted to check what impact the babel plugin had on a real website, just checking the size of the .next/static folder after running yarn docs:build it's about 56kb smaller.
Using yarn size:snapshot and http://webpack.github.io/analyse it's 2kb