docs(rfc): scaffold rfc-submission workspace

[mojoatomic]

Scaffolds rfc-submission/ with:

• README.md (overview, tooling, structure)
• missing-sections.md (Abstract, Security, IANA, references outline)
• rdcp-spec.xml (xml2rfc v3 skeleton)
• submission-checklist.md (process steps/tools)
• CHANGELOG.md (draft iteration log)
• reviews/.gitkeep

Refs #31

[mojoatomic]

Update:

• Added Abstract, expanded Security Considerations, IANA Considerations, and references to rfc-submission/missing-sections.md (aligned error codes to spec/schema).
• Fixed xml2rfc skeleton placeholders (docName/author).
• Validated with xml2rfc (text+html generated). Current result: renders successfully with expected citation warnings since the body does not yet cite references.

Next steps:

• Merge this scaffold PR, then port content from missing-sections.md into rdcp-spec.xml sections and add citations.
• After filling endpoints and citing references, xml2rfc should be warning-clean.

Refs #31