Add an Actor authoring contract page: supported schema drafts and current actor.json keys

[patrikbraborec]

Context

No single page states which JSON Schema drafts input schemas support, or which actor.json keys are current versus deprecated. An Actor written from the docs alone can carry a schema the platform rejects at build time, which is what caused a failed build in a recent agent run.

To do

• One page listing the supported JSON Schema draft(s) for input and storage schemas, and what happens when an unsupported draft is used.
• A table of actor.json keys: current, deprecated, and removed, with the version each changed in.
• Link it from the Actor definition, input schema, and Actor development pages.

Done when

One page answers what may go into a schema and into actor.json, without trial builds or reading platform source.