Update IPNS specification

[lidel]

• Update IPNS record section to document support for extensible records (and v2 signature), matching implementation in GO and JS
• Explain v2 signature generation and validation
• Make it clear that "Routing record" key is /ipns/{binary_blob} so implementers and devs don't think it is a bug 🙃
• Make it sure specs are aligned with implementations such as Kubo and JS-IPFS
• Move IPNS.md and naming/pubsub.md to ipns directory
• Triage https://github.com/ipfs/specs/issues?q=is%3Aissue+is%3Aopen+ipns
• Incorporate any insights from https://github.com/aschmahmann/ipns-utils and How-To Crypto notes#249