docs: add draft IANA formal URN namespace registration for substrait

[nielspardon]

Drafts the RFC 8141 Appendix A registration template to register substrait as a formal URN namespace with IANA, so our extension identifiers become valid URNs. Per RFC 8141 §6.2 the template is reviewed by emailing it to urn@ietf.org (Expert Review) — a published RFC is not required.

The registered form follows what we converged on in #881:

urn:substrait:extension:<owner>:<id>
e.g. urn:substrait:extension:io.substrait:functions_arithmetic

substrait is the IANA-registered NID; the extension resource-type token leaves room to add other Substrait URN types (and an optional version segment) later without a second migration. The template specifies the syntax as ABNF plus an equivalent regex, along with assignment (open, delegated to reverse-DNS ownership), equivalence, security/privacy, interoperability, and resolution (none mandated — location-independent identifiers).

Opening as a draft to gather maintainer input on two things:

1. Do we want to keep the registration doc in the repo at all, or only track it externally as part of the submission?
2. If we keep it, where should it live — repo root (current), site/docs/, or a proposals//governance directory?

Still open before any actual submission: filling in a durable Registrant point of contact, and confirming we want to register now vs. after the URN format (incl. versioning) stabilizes for 1.0.

Towards #1016.

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