Better document Multi-Modal Annotation (Compound File) #1553

Existing Context
https://diffgram.readme.io/docs/multi-modal-annotation
https://diffgram.readme.io/docs/conversational-annotation

• One of the strong features of Diffgram
• Many concepts like global attributes
• Continually adding more interfaces into this concept (e.g. audio etc)

Existing docs

• Missing introduction prose / context
• Missing links, e.g. explaining how global attributes scope relates
  https://diffgram.readme.io/docs/attribute-scope
  how to query in compound context etc.
• Missing screenshots or video example
• General organization of Compound file relate features and getting started concepts