Document horizon deprecation vision/plan

[mollykarcher]

Internal to the SDF, we are aware that Horizon is not the model that we would like new people to onboard onto. However, our long-term vision is not broadly understood by the wider ecosystem. We've talked about this with partners, published blogs, and talked publicly, but it would help to have a concrete link to point people to in the docs that communicates this. This would help in that developers just starting out and exploring the docs would be more aware of it, and it would also serve as a place for us to point ecosystem members to when questions around our stance come up.