managed-instances: deploy a demo instance

Created by: bobheadxi

after https://github.com/sourcegraph/sourcegraph/issues/13120 and related changes dogfood will likely become a rather unstable deployment. it might be valuable to add a more stable deployment that actually tracks releases for demo purposes (probably useful for @sourcegraph/customer-engineering)

This could be a good chance to take @slimsag 's extensive managed instances documentation for a spin too, and hopefully won't be too much overhead. I imagine it might work as a mini-testbed to try out changes before changing customer deployments too.

cc @pecigonzalo