Document how to replace the default Phoenix stack with just Bun

[crbelaus]

A great idea was shared on #20 that consists on replacing the default Phoenix stack (esbuild + tailwind packages) with just Bun.

This makes a lot of sense and I believe would be a great addition to the documentation. We could have a section dedicated to how to make this change and how to configure bun build as an esbuild replacement and bun run tailwind as a tailwind replacement.