Documentation

[andreasgangso]

• Comment all API functions, at the very least in BccmPlayerController.
• Move everything from README.md to doc/
• Clean up top level exports, some folders like theme/ should be in lib/ and exported via bccm_player.dart for example. This also cleans up the API reference docs.
• Document a few common scenarios as examples
• Document how to customize the player (theming and custom ui)