Update docs for custom primitives

[jeff-hernandez]

The documentation for defining custom primitives was updated to use the class-based approach instead of the function-based approach. The updated pages are listed below:

• Feature Primitive
• Advanced Custom Primitives Guide

[codecov]

Codecov Report

Merging #1332 (7272333) into main (6f7cead) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1332      +/-   ##
==========================================
+ Coverage   98.38%   98.58%   +0.19%     
==========================================
  Files         135      135              
  Lines       14496    14516      +20     
==========================================
+ Hits        14262    14310      +48     
+ Misses        234      206      -28     

Impacted Files	Coverage Δ
featuretools/computational_backends/utils.py	95.54% <ø> (ø)
...mputational_backend/test_feature_set_calculator.py	100.00% <ø> (+0.36%)	⬆️
...uretools/tests/entityset_tests/test_es_metadata.py	100.00% <ø> (+0.59%)	⬆️
...eaturetools/tests/entityset_tests/test_plotting.py	100.00% <ø> (+1.63%)	⬆️
...ols/tests/synthesis/test_deep_feature_synthesis.py	99.29% <ø> (+0.13%)	⬆️
featuretools/utils/plot_utils.py	92.00% <ø> (ø)
...s/computational_backends/feature_set_calculator.py	98.70% <100.00%> (+0.01%)	⬆️
featuretools/entityset/entityset.py	98.77% <100.00%> (+0.81%)	⬆️
...utational_backend/test_calculate_feature_matrix.py	99.44% <100.00%> (+<0.01%)	⬆️
featuretools/tests/conftest.py	100.00% <100.00%> (+3.40%)	⬆️
... and 7 more

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[rwedge]

In the advanced custom guide, let's add an example of writing custom column names for a primitive with multiple output columns

[rwedge]

Looks good!