Remove autoprefixer from Parcel installation guide

[devongovett]

Parcel v2.4.0 now uses @parcel/css by default for auto prefixing and transpilation (like postcss-preset-env). It shows a warning telling users to remove autoprefixer from their postcss config file, which is now redundant with the builtin auto prefixing support and can improve performance.

Unfortunately, it looks like the tailwindcssCLI doesn't have a way to skip autoprefixer when generating a postcss config, so I removed the -p option and created the postcss config containing only tailwindcss manually in these instructions for now.

It also seems that autoprefixer is a peer dependency of tailwindcss, and you do get a warning during install when it isn't added. Maybe that could be made optional instead?

Also switched from a postcss.config.js to a .postcssrc which enables Parcel's caching to work better.

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[adamwathan]

Thanks man! 🙌 I've opened a PR to kill the peer-dependency as well, so will have that sorted in the next patch: tailwindlabs/tailwindcss#7949