Document operator workflows

There are various workflows that the operator enables. Some steps are carried out by the user (creating `daskcluster` resources), some things are done by the operator (creating pods/services), some things are recursive (the operator creates worker groups, then the operator creates pods for those worker groups) and other things are done by kubernetes itself (deleting a `daskcluster` causes kubernetes to cascade delete the worker groups, pods, services, etc).

It would be good to document all of these things. Something like this.

### Installation
- user installs new dask cluster and worker group resource types
- user installs operator daemon

### Cluster creation
- User creates cluster resource
- operator notices cluster resource and creates scheduler pod/service and worker group resource
- operator notices worker group resource and creates worker pods

### Cluster scaling
- User modifies cluster worker count
- operator notices change and starts/stops pods to match

### Cluster adaptive mode
- User toggles adaptive setting on the cluster resource
- operator begins polling the scheduler for desired number of workers and adjusts the worker count on the cluster resource to match (triggering the scaling workflow when it changes)

### Cluster deletion
- User deletes cluster resource
- Kubernetes cascade deletes all child resources including worker groups, pods and services

### Create additional worker groups for heterogenous clusters
- User creates new worker group resource (with different resources to the default like GPUs or high memory)
- operator notices new worker group and creates pods
- operator adopts the worker group resource to the cluster resource so that it will also be cascade deleted

_This could also be a fun time to try out [mermaid diagrams](https://mermaid-js.github.io/mermaid/#/)._