Provide Kubernetes manifests for deploying Briefer

[lucasfcosta]

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.

[IonesioJunior]

Could someone assign this to me?