Add design doc for a manual annotation tool

[kabilar]

• [x] After discussion with Anastasia and Hong-Hsi, we have defined 2 use cases.
• [x] I have collated the requirements from the milestones final document and the above use cases. My local copy of this design doc matches each requirement to a milestone number. I have not pushed these milestone numbers to GitHub since we probably don't want specific information on the milestones to be public.
• [ ] Evaluate possible solutions and add a description of the evaluations to this document

cc @aaronkanzer @ayendiki @satra

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