Document differences between protected and non-protected participant-level TSV

[alyssadai]

Since the changes made to the API and query tool, the participant-level results tsv file will now include protected literal as value for columns like age, sex, diagnosis, etc.
So we want to mention this change here in our docs.

• Mention participant-level results tsv will be different when NB_RETURN_AGG is set to true
• Add an example file to neurobagel examples and link it in the docs