add API documentation

[rdeits]

No description provided.

[codecov-commenter]

Codecov Report

Merging #164 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #164   +/-   ##
=======================================
  Coverage   78.89%   78.89%           
=======================================
  Files          18       18           
  Lines         507      507           
=======================================
  Hits          400      400           
  Misses        107      107           

Impacted Files	Coverage Δ
src/MeshCat.jl	80.00% <ø> (ø)
src/abstract_visualizer.jl	100.00% <ø> (ø)
src/animations.jl	80.00% <ø> (ø)
src/arrow_visualizer.jl	100.00% <ø> (ø)
src/assets.jl	0.00% <ø> (ø)
src/atframe.jl	85.29% <ø> (ø)
src/objects.jl	100.00% <ø> (ø)
src/servers.jl	3.57% <ø> (ø)
src/trees.jl	96.29% <ø> (ø)
src/visualizer.jl	90.66% <ø> (ø)
... and 1 more

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