Follow-up tasks related to the new git-based bundles implementation merged as part of 0.2.

https://github.com/operator-framework/rukpak/issues/204

A/C

• Refactor git-based e2e tests to be more idiomatic
• Add some documentation on git-based bundles, primarily examples
• Examine how to possibly incorporate CRD-schema level validation for the union type semantics exposed through the new source field. If not possible, move git-based bundle level validation into the bundle webhook.