Add docs for Pulumi ESC provider for Kubernetes' External Secrets Operator (ESO).

[thoward]

Documentation of the provider exists here: https://external-secrets.io/latest/provider/pulumi/

But we have nothing on our site about this. Extend docs at https://www.pulumi.com/docs/esc/integrations/kubernetes/ to include, at minimum, a mention of this Provider and what features it supports, with a link to the ESO docs.

Stretch would be to write a tutorial about how to use it.