doc(widget): server API docs (#443)
doc(widget): server API docs (#443)

A docstring to help widget implementers. Please consider this a first draft.

Co-authored-by: Ed Ayers <5525be1fdf6221ee30be52359259248ffc35fe1a@users.noreply.github.com>

dselsam/lean,leanprover-community/lean,leanprover-community/lean,dselsam/lean,digama0/lean,digama0/lean,dselsam/lean,leanprover-community/lean,digama0/lean,leanprover-community/lean,dselsam/lean,leanprover-community/lean,dselsam/lean,dselsam/lean,digama0/lean,digama0/lean,leanprover-community/lean,digama0/lean