Update Handling Time Docs

[Seth-Rothschild]

Feature Request Description

We should add to the handling time docs so that they're a better resource for users. This includes

• 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

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[kodonnell]

Add visuals which show how we use time

Definitely. In general, I've found that most people (myself included) misunderstand the complexity of handling time overlaps etc., and that diagrams are the best method for clarifying this. It'll also make me (as a user of featuretools) much more confident that you're doing it 'right' (which would be awesome so I don't have to worry about it, as it's always the hardest part!).