feat: add v1beta3 (#34)

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Author: yoshi-automation

document_ai/AUTHORING_GUIDE.md

@@ -0,0 +1 @@
+See https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md

document_ai/CONTRIBUTING.md

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+See https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/CONTRIBUTING.md