docs: Operator example notebook & api reference docs

[tchughesiv]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Misc

[dmartinol]

Very exhaustive showcase of the main operator deployment functions.

• I would just add a short explanation of the feast architecture (a Topology diagram?)
• maybe also clarify what the client pod does under the scenes with the init-containers, maybe adding the relevant snippets or short text description:

until grpcurl -insecure -d '''' -format text feast-example-registry.feast.svc.cluster.local:443 grpc.health.v1.Health/Check; do echo waiting for registry; sleep 2; done

#feast init
with mock.patch("click.prompt", side_effect=["postgres.feast.svc.cluster.local", "5432", "feast", "public", "feast", "feast"]):
  with mock.patch("click.confirm", side_effect=[True]):
    init_repo(project_directory, template)' > postgres-apply.py;
...
feast apply
...
CURRENT_TIME=$(date -u +"%Y-%m-%dT%H:%M:%S");
feast materialize-incremental $CURRENT_TIME

[dmartinol]

can we add the few more kubectl exec commands to run sample commands like feast XYZ list to prove the repo was correctly created?

[dmartinol]

It seems these notebooks have no output. While I agree that maintaining outputs in git can be challenging, having the outputs preserved is indeed very helpful when the notebooks from a browser. It provides immediate insight into the results of the experiment without needing to rerun it. WDYT?

[tchughesiv]

oh ok, i hadn't thought of it that way. i'll add them

[dmartinol]

lgtm!