Adding JTBD examples and tips

Why is this change being made?

We have a lot of documentation around how to structure JTBDs, what they are, what they aren't, etc, but don't provide many examples of what they look like / how to iterate through feedback. I've been re-writing a lot of them lately, most recently with performance testing, and worked with @rayana and @enf to iterate. I still continue to learn more about JTBDs as I run more research and rewrite them, so I think it'd be helpful to share an example of what that looks like. This could also be helpful for someone who has never written a JTBD before!

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