[ENH] Add help text for built-in datatable filtering syntax and show column datatypes

[alyssadai]

• Closes [ENH] Enable better filtering for missing values #112
• Closes [ENH] Improve documentation of built-in filtering options #113
• Closes Switch to using app.run in development #119

Changes proposed in this pull request:

• Add collapsible help text for datatable filtering syntax, including filtering for missing values
  • Note that filtering for non-missing values will not be supported until Switch to using AG Grid for interactive table #127, since it would likely involve switching entirely to custom backend filtering if using dash datatable
• Switch to not using dcc.Markdown where possible to avoid un-adjustable extra bottom padding
• Set column type explicitly in datatable based on pandas dtype, and display in phenotypic column summary (allows users to have an easier time choosing the right filtering operators)

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[surchs]

Very nice PR @alyssadai 🎉! With the amount of custom callbacks and style handling you're doing, you'll feel right at home in the frontend once you'll join us there.

I left one comment to add a disclaimer that a "is not blank" filter does not (seem) to exist - because that's the first thing I tried after learning about "is blank".

🧑‍🍳