services/horizon: Update README docs for installation types

[sreuland]

What problem does your feature solve?

The main README touches on developer install and production install.

What would you like to see?

The README should focus on just stating the preferred ways to build and/or install horizon for development purposes. State the options, like using quickstart, or just the horizon dockerhub image or building from source. Mention any multi-arch gotchas, such as if arm64 is not supported or how to use in platform safe way.

Remove any docs related to production installations, and refer to public in install docs instead.

What alternatives are there?