Support :ref :cite :defn  #14

Would be cool if we had built-in support for referencing specifications using high resolution references that already exist at standard bodies like W3C.

FYSA @darobin started poking at this problem space for stable referening in IPFS specs using :ref :cite and :defn.

• Early prototype in https://github.com/darobin/ipseity
• Sample usage: https://raw.githubusercontent.com/darobin/ipseity/main/specs/meta/spec-for-specs.md → https://bafybeigv7dr3rom6uzxehh6tbfskuisnbo3ymx35urc2fifmdw36o5wdb4.ipfs.w3s.link/meta/spec-for-specs/#refs

I think what we want docs websites to support :ref :cite and :defn too – this way we can refer to specific field in spec using stable pointers, or do cross-site linking (ipfs docs linking to pubsub description in libp2p docs etc)