Merge k8s docs into main docs site

Created by: beyang

Kubernetes Sourcegraph site admins have been confused when they go to look up how to do something (e.g., configuring external Redis and Postgres) and find the main docs site page rather than the deploy-sourcegraph one.

A Google search for "sourcegraph external redis" yields the first page at the top and the second one 4th from the top.

We should have a single place for all core documentation.