docs: add Nebula wire protocol DocC spec

[gradyzhuo]

Summary

• Add swift-docc-plugin to Package.swift and create Nebula.docc catalog with @TechnologyRoot landing page
• Write 5 DocC articles: WireFormat, MatterTypes, BehaviorCatalog, NodeBehavior, ConformanceTests
• Add 10 conformance test JSON files (one per behavior: Register, Find, Mutate, Get, Unregister, Enqueue, Ack, Subscribe, Unsubscribe, Event)
• Add GitHub Actions workflow to build and deploy documentation to GitHub Pages on push to main

Test Plan

• All 56 existing tests pass (swift test)
• swift package generate-documentation --target Nebula builds successfully
• Enable GitHub Pages in repo Settings → Pages → Source: GitHub Actions
• Push to main triggers workflow and documentation appears at https://offskylab.github.io/swift-nebula/documentation/nebula/

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