Add GitHub Actions job to deploy API documentation

[klauer]

• Preliminary sphinx docs are now in with #89
• Need to add a GitHub Actions job to build and deploy those docs to GitHub Pages
  • docs-versions-menu usage would be nice
• Separate but related - more docs need to be written (#52)

[klauer]

This is now working after #92

Sample page: https://klauer.github.io/blark/master/api/blark.transform.VariableDeclarations.html