feat(docs): add DocFX parser for Cloud RAD documentation

[bshaffer]

The DocFX Parser is called using the google-cloud utility:

$ dev/google-cloud docfx --component Vision --metadata-version 1.0.2 --staging-bucket docs-staging-v2-dev
Generating documentation for Vision
Running phpdoc to generate structure.xml... Done.
Writing output to out... Done.
Writing docs.metadata with version 1.0.2... Done.
Running docuploader to upload to staging bucket docs-staging-v2-dev... Done.

This generates a .phpdoc/build/structure.xml file in the CWD using the phpdoc CLI, which is a well established documentation format that is then injested and parsed into the DocFX YAML format.

The result of that is then compressed and uploaded to GCS using gcs-uploader, and Cloud RAD takes it from there.