Non-doc changes

• allow empty package or path in AssetId.parse since the main constructor allows them, add test coverage showing it allows them
• tweaks to error messages
• add test coverage to changeExtension so I am sure I documented it correctly :)
• rename test for asset_id.dart: id_test.dart -> asset_id_test.dart

Doc / readability improvements

• rewrite for clarity / brevity
• some corrections about which exceptions can be thrown
• move members around to put most important ones
• move AssetReader and AssetWriter next to BuildStep and the docs on BuildStep since that's the class builders interact with directly
• add docs to BuilderFactory and BuildConfig

I didn't get to all the API yet, more to come later :)