Move "subgraph" to supported repository

[BigLep]

Done Criteria

1. This repo's subgraph is moved to a repo that is properly supporting it.
2. There are pointer(s) from this repo about the subgraph location.
3. There are pointer(s) from public docs to the subgraph location.

Why Important

FilOzon/filecoin-service maintainers aren't consumers of this subgraph and thus don't have an inherent knowledge on what the subgraph needs to expose nor the vested interest to host it. This leads to a bad experience for consumers as they find a half-based subgraph.

Notes

1. It was decided in 2026-02-03 FOC WG that FilB will take this over.
2. This was also discussed in slack in https://filecoinproject.slack.com/archives/C08TVNKJV7C/p1767914301013149