Improve documentation for Build resource

[mvalliath]

We've gotten feedback that it would be helpful to provide users with documentation on how the Build resource can be used to achieve a one-off build experience, similar to pack build

We should provide documentation on how users can leverage the build resource to provide an imperative build experience.

Note: Some users of the Build resource are likely pack users looking to the Build resource as a way to get started with kpack. As such, we should make this documentation as beginner-friendly as possible :)