tutorial: Demonstrate "footnotes" xref style
Apropos #8 (closed), another possible approach. See !196 (merged) for rubric. In this MR the principle is:
In each subsection, add a "Reference" footnotes bit, which links to the reference documentation for the features being introduced/discussed.
FTAOD this MR and !196 (merged) are mutually exclusive; or there may be a better way.