Add specification for sources

The [source specification](https://github.com/knative/eventing/blob/main/docs/spec/sources.md) hasn't been moved to the repository (yet).

At the minimum the specification should include the source duck type (in the document linked above).

I believe specific sources, such as PingSource, ApiServerSource, are out-of-scope, as I don't except multiple implementations of those (except single-tenant vs multi-tenant variants). If someone think otherwise, please add a comment below.

From a conformance PoV, I wonder if we should introduce a new `Source` CRD, similar to `Channel`, delegating the actual behavior to underlying implementations. Obviously the notion of default source does not exist. Anyway, something to think about.

Start by comparing the linked source specification with the repository's existing specification documents. Identify the source duck type requirements that must be documented, then check the issue's proposed scope around specific sources and a Source CRD before proceeding. Done means the source specification is added to the repository with the agreed scope and required duck type.

Written by the indexing model from the issue text.