AS2-Vocab: Split "conformance" to its own section

[trwnh]

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", " SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Instead of this bespoke text, we can rely on ReSpec boilerplate auto-generated from <section id="conformance"></section>.

[evanp]

Yeah, we shouldn't do this by hand.