Add Fargate install docs for Sourcegraph

Created by: KattMingMing

A user requested documentation for how to install Sourcegraph via Fargate.

We have: https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/k8s.eks.md - I don't have much experience with Fargate, but could we leverage any of our k8s deployment for this?

Requested by https://app.hubspot.com/contacts/2762526/company/1148979440