Expose pinning service api spec

[BigLep]

Done Criteria

Minimum: there is a pointer from https://specs.ipfs.tech/ to https://ipfs.github.io/pinning-services-api-spec/

Why Important

Further solidifies specs.ipfs.tech as the canonical place for IPFS specs.

User/Customer

Someone looking for the surface area of IPFS-related specs. (I assume though that anyone who already knows about the pinning service api will be able to find it easily enough without this.)

Notes

1. I know the pinning service is not a major area of investment, but I was surprised that I didn't see it at https://specs.ipfs.tech/ and had to remember it was elsewhere. I assume there was discussion about this in the past, but I couldn't quickly find it. If nothing else, this issue can serve as a pointer even if don't do the work.
2. I'm imagining adding another block after "Content Filtering" at https://specs.ipfs.tech/ for "Pinning", and from there can link out to https://ipfs.github.io/pinning-services-api-spec/ for now.
3. This seems like an easy issue to assign to an "Agent" to create a PR for. I am happy to do this if it's even wanted - fine if not.