[Docs] Update YAML reference doc to include properties of builtin pulumi variable

[scottslowe]

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Issue details

Per @AaronFriel, there are three properties exposed on the builtin pulumi variable: pulumi.cwd, pulumi.project, and pulumi.stack. However, these properties do not seem to be documented in the Pulumi reference documentation.

Affected area/feature

The Pulumi YAML documentation

[aq17]

Closed via https://github.com/pulumi/pulumi-hugo/pull/2344