[ENH] Document descriptive and machine-optimized query result TSVs

[alyssadai]

• Closes Update the relevant sections to reflect the changes made to query tool result files #244
• Related Merge aggregate and non-aggregate dataset examples in query result TSVs neurobagel_examples#45

Changes proposed in this pull request:

• Update "Downloading query tool results" section to reflect new query result TSVs
  • Move example result TSVs into expandable admonitions
• Switch to tables for column descriptions (for reviewer: thoughts on storing these in TSVs in this repo?)
• Update section on aggregate query results to clarify possibility of both aggregate and subject-level data in a query result TSV

TODO: Confirm that query result TSVs look correct in preview after neurobagel/neurobagel_examples#45 is merged

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[alyssadai]

@neurobagel/dev, I'm going to go ahead and merge this for now to unblock our BIC node redeployment, and for the sake of keeping the docs up-to-date with our deployed tools (this PR is related to neurobagel/query-tool#370).

That said, I'd still appreciate some feedback on the new text / tables when you have a chance!