Document: Update docs as Substreams-Powered Subgraphs are being deprecated

[BigtoC]

This is the PR to remove SpS, it's released in v1.15.7.
And in the document (https://thegraph.com/docs/en/substreams/sps/introduction/), the link https://docs.substreams.dev/how-to-guides/sinks/subgraph/graph-out is no longer present in the substreams documentation.

After SpS is deprecated, what is the replacement for SpS?