Document all SPIs

[zml2008]

Adventure has grown quite a few SPIs that platforms should be implementing. We should have a consolidated location where these are documented. So far, these are:

• the Providers for component serializers, to customize options
• the JSONComponentSerializer providers, for different component serializer implementations
• the DataComponentValueConverter.Provider interface, for controlling conversions between different data component value types
• the ClickCallback provider, to implement callbacks
• ComponentLoggerProvider to provide appropriate serializers for component logging