[Docs]: Add on documentation setup of postcss

[xeinebiu]

I tried using the project today and struggled for a while because no styles were processed, even though I followed the documentation correctly: Getting Started Guide.

I suggest adding a section on setting up postcss.config.js to help new users have a smoother experience.

PostCSS Configuration:

// postcss.config.js
module.exports = {
  plugins: [require('@pandacss/dev/postcss')()],
}