Document Java language extensions

[t0yv0]

Hello!

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Issue details

Would be good to document what is available for .language.java in the Package Schema:

https://www.pulumi.com/docs/guides/pulumi-packages/schema/#language-specific-extensions

This change should go into pulumi/docs.

I'd also want us to give this struct some more prominence since it's our typed model of the same:

// PackageInfo represents a Java language-specific info for a package.
type PackageInfo struct {
	Packages    map[string]string `json:"packages,omitempty"`
	BasePackage string            `json:"basePackage"`

	// If set to "gradle" generates a basic set of Gradle build files.
	BuildFiles string `json:"buildFiles"`
}

Like let's lift it to its own file and cross-link to the docs. Thank you.

Affected area/feature