Publishing Pulumi Packages: Document schematized configuration, environment variable mapping for providers

[AaronFriel]

A user authoring a provider may not realize that we support schematizing configuration keys, which can enable reading config and secrets from env vars or Pulumi config. We should document this in a page such as the Pulumi packages how-to-author page, or, along with schema information (#2955) move it into the boilerplate repositories.

Bridged providers for example have their own code-level method of declaring configuration keys.