[MNT] Remove deprecated Nipoppy docs

[alyssadai]

• Closes Deprecate or remove Nipoppy docs in favour of readthedocs? #218

Changes proposed in this pull request:

• Recommendation to explore Nipoppy added to https://neurobagel.org/data_prep/
• Minimum version of mkdocs-table-reader-plugin added to avoid pandas DeprecationWarning on mkdocs serve

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[surchs]

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