Add DNSLink specs for resolving /ipns/{fqdn} content paths

We have a dedicated https://dnslink.dev website with loose specification, but it lacks necessary maintenance and love.
It is also written from IPFS-agnostic perspective, and does not go too deep into how `/ipns/{fqdn}` and `/dnslink/{fqdn}` resolution should work in IPFS ecosystem specifically. Due to this, our stack still only supports the former, and the latter can't be used, blocking people from using DNSLink for more than just IPFS.

Given how important DNSLink is for IPFS ecosystem, we should write proper spec that describes how `/ipns/{fqdn}` should be resolved and add it to https://specs.ipfs.tech. This way we close the specification gap, and have specs for all identifier types on `/ipns/` namespace.