Edits to parts 1 and 2 of market intelligence use case

[frreiss]

This PR contains the next batch of work on the market intelligence tutorial/blogs:

• Minor edits to part 1
• Redo code snippets for part 1 as Github gists
• Major edits to part 2
• Github gists for code in part 2
• Initial very rough draft for part 3

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[frreiss]

@BryanCutler would you mind having a look at the current version of Part 2?

[frreiss]

Merging these changes. Will come back with a new PR and address @BryanCutler's comments above on clarifying the transitive closure parts.