Provide Kubernetes manifests for deploying Briefer

We need to provide users with manifests for deploying Briefer's open-source version into their Kubernetes clusters.

We should use our `docker-compose.yml` file as a starting point for that.

Here's what needs to be done:

1. Create a separate folder for Kubernetes manifests - it'll contain all the necessary `.yaml` files.
2. Create manifests for each service. That includes deployment specs for the `web`, `api`, `ai` and statefulset specs for `jupyter`, and `postgres`.
3. Remember to create the necessary services for each.
4. Remember to update `docs/DEPLOYMENT.md` with the instructions.

We should get this working on minikube first.

We'll worry about a helm chart later.