Improve Go Mobile Docs

[thereisnogabe]

          I think it's important to cover:

• Documentation on how to include this in an android/ios project, with a simple example.
• CI that exercises actual Android/ios code so we can be confident this works.
• Guidance to readers that tells them how to discover available functionality within the SDK.

Of course, that's a lot of scope for this PR, so tracking in separate issues is cool. But I think this is critically important.

Originally posted by @andresuribe87 in #457 (comment)