Integrate Dokka into all SDK components and build API reference docs

[williamito]

Describe the Issue Kotlin's documentation generation tool is called Dokka. API reference docs should be available for developers, and rebuilding them should be a part of every release. Looking at the diff in generated reference docs between versions is a strong tool for understanding changes between versions. Code-level documentation is also tightly integrated with Android Studio, e.g. shows up when you hover over components in the code. Generating docs would also help identify where documentation is lacking or not keeping up with style guide expectations.

[williamito]

I created a PR that adds simple configs for Dokka. In addition I think we could:

• Add a doc generation step to release process
• Check in the generated documentation
  • This should only be updated with releases, i.e. the reference docs reflect the most recent release and not head
• Enable Github Pages pointing to the location where documentation is checked in to render the reference docs

Would love feedback on the PR and a discussion on how to proceed.