Add YARD documentation to release-toolkit actions

Add YARD documentation to all the methods in release-toolkit, especially the methods on actions and helpers.

Part of this task will likely be tackled in parallel to the code rearrangement / refactoring in #191 – as we plan to write YARD documentation for new code as it is being refactored / rewritten. But this issue will cover more than #191 as we need to also document even code that won't really need to change / be refactored.

• Helpers
  • *_version_helper iOS + Android (🚧 WIP)
  • *_git_helper iOS + Android (🚧 WIP)
  • Other iOS helpers (🚧 WIP)
  • Common (non-platform-specific) helpers
• Actions
  • Android actions
  • iOS Actions
  • Common Actions
  • Configure Actions

Start by reviewing the Ruby helpers and actions in release-toolkit alongside the code rearrangement described in #191. Work through the listed iOS, Android, common, and configure areas, and consider how refactoring affects the documentation. Done means the methods in all listed categories have YARD documentation, including code that is not refactored.