Handling time rewrite

[Seth-Rothschild]

close #244

• Add a substantial section on what time_index means (with retail example)
• Add a section for secondary_time_index with the flight example
• Add example with same index but multiple time indices
• Emphasize that cutoff times are an instruction list and limit the row output
• Emphasize the ways that time is used in building features
• Merge Training Window and Last Time Index sections/examples
• Add approximate paragraph from elsewhere in docs
• Add visuals which show how we use time

Draft available here as html:
handling_time9-24.zip
(updated 9/24 12:03)

[codecov-io]

Codecov Report

Merging #245 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   94.45%   94.46%   +<.01%     
==========================================
  Files          71       71              
  Lines        7670     7676       +6     
==========================================
+ Hits         7245     7251       +6     
  Misses        425      425

Impacted Files	Coverage Δ
...utational_backend/test_calculate_feature_matrix.py	99.28% <100%> (ø)	⬆️
featuretools/demo/mock_customer.py	95.45% <100%> (+0.71%)	⬆️

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[Seth-Rothschild]

@kveerama @kmax12 this should be good to go

[kmax12]

Looks good merging