Stand up yard docs

Replaces https://github.com/calabash/calabash-android/pull/471 which had merge conflicts.

@TobiasRoikjer Let's pair program tomorrow and resolve the WIP commit 3b71fb4 and ~~PR #469~~

### TODO

- [x] @TobiasRoikjer or someone on the Android team needs to update ENVIRONMENT_VARIABLES.md
- [x] Should the lib/**/*.md files be included in the docs?  Can they be removed from the repo?
     - The file in question is ./lib/calabash-android/canned_steps.md Keep it.