update k8s docs

Summary:
for the deploy_k8s example, we can only remove the .mdx and not the corresponding code yet since the code is used to create the dagster/k8s-example

https://dagster.slack.com/archives/C014N0PK37E/p1610837743002500
https://dagster.slack.com/archives/C014N0PK37E/p1610837483001500

Test Plan: bk

Reviewers: nate

Reviewed By: nate

Differential Revision: https://dagster.phacility.com/D6037