Context document versioning

[evanp]

We maintain a set of documents with version stamps, for processors that need the exact version of a context:

https://www.w3.org/wiki/Activity_Streams/Primer/Context_document_versioning

Should we mention this in section 2.1?

[trwnh]

Related issues:

• Link from rolling version of the AS2 context to the latest numbered version #579
• Update version document Primer page to include full versioning process #589
• Mention versioned contexts and add a changelog to w3.org/ns/activitystreams #659
• Consider using a version parameter on the IANA media type application/activity+json #661

I think this might be a functional duplicate of #659.

[evanp]

In Issue Triage, we decided to split the work between updating AS2 Core doc and updating the namespace document into two issues. So, this issue is specifically about making a change to AS2 Core to note the version policy.