Decide where example-data-fetching should live

[mpharrigan]

1. Put download functions in the same place (documentation_utils.get_qaoa_data and documentation_utils.get_fermi_hubbard_data).
2. Put download functions in respective modules (qaoa.get_publication_data and recirq.get_publication_data).

Originally posted by @rmlarose in #117 (comment)

We should pick one and make it consistent