Update demo data sets to be more user friendly

[kgodey]

Came out of testing our demo data sets.

@pavish to flesh out more.

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Figure out a strategy for the data sets, i.e., what are users supposed to understand from them, and implement that strategy. The NOT NULL thing is kind of a symptom.

• Are these data sets supposed to be something they just think about at a "hey, I could manage inventory!" level, or
• Are these data sets supposed to demonstrate various specific features of Mathesar.

If the former, we should just remove the NOT NULL constraints (and lots of the check and other constraints). If the latter, we should make sure they're covering all features they should, and avoid or at least heavily explain the examples of using underlying PostgreSQL features not accessible directly through Mathesar. Etc.

If we keep the constraints, we should ensure that they are explicitly mentioned and users are walked through them in the guides.