Add documentation for Bun

[wout]

This is to document luckyframework/lucky#2015 and luckyframework/lucky#2020. The same goes for this implementation as I mentioned in #1522, if it would help to evaluate the whole thing, I can already write the documentation before pulling it in.

• Configuring Bun
• Plugins for Bun
  • Built-in plugins
  • Writing CSS and JS plugins
  • Writing custom plugins
• Migrating from Laravel Mix to Bun

I don't have a project still using Mix, but I could upgrade this repo (the Website) to Bun as an exercise and reference for the documentation.