Beta documentation: write up alternatives to PipelineResources if they remain Alpha

Expected Behavior

Clearly documented approach to passing files, folders, docker images, cluster information, auth information, repo information etc etc without relying on PipelineResources.

Actual Behavior

At the moment our docs assume that PipelineResources are "the answer" to modelling meaningful resources external to the CI/CD system. In the most recent PipelineResource working group we landed on the idea of keeping PipelineResources as Alpha when the rest of the Tekton resources move to Beta. So with that approach we'll need to make sure that our docs correctly reflect the "right" way to work with external resources.