docs: initial (not-working) version of DocFX YAML generation

[tbpg]

This line fails in class.yml.twig:

- uid: {{ node.fullyQualifiedStructuralElementName|yaml }}

You can try it locally with:

cd Vision
composer update
rm -rf docs && phpdoc --template=data/templates/docfxyaml/ -t docs -i vendor

See https://docs.phpdoc.org/3.0/guide/guides/templates/twig.html.

cc @bshaffer