Add feature descriptions to feature graphs

[frances-h]

Add ability to add a description of the feature to the feature's graph as a graph caption. A description can either be manually passed in, or graph_feature can automatically generate a description with ft.describe_feature:

[codecov]

Codecov Report

Merging #1212 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1212   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files         134      134           
  Lines       14309    14337   +28     
=======================================
+ Hits        14114    14142   +28     
  Misses        195      195           

Impacted Files	Coverage Δ
featuretools/feature_base/feature_visualizer.py	98.61% <100.00%> (+0.04%)	⬆️
...s/tests/primitive_tests/test_feature_visualizer.py	100.00% <100.00%> (ø)

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[rwedge]