Document pod-overrides and container-overrides

[kadel]

Which area is this feature related to?

/area documentation

Which area is this documentation change is related to?

/area api
/area library

Issue details

pod-overrides and container-overrides were implemented in devfile/library#158 and devfile/library#157

but there is no documentation on devfile.io about how to use it and what is it for

context in #920

[valaparthvi]

Thank you for opening this. I am planning on working on this. I was waiting for both the PRs to go in.