Use dokka for reference docs

[sarahkoop]

Summary of changes

• Add dokka plugin for reference docs generation
• These docs are generated and added to the docs directory in this repo (no need to review those auto generated files).

• The documentation will live in github pages similar to braintree_android once the page is created (future TODO).

  Checklist

  • ~[ ] Added a changelog entry~

Authors

• @sarahkoop