Doc: use "unnumbered" footnotes (GH-98954)

Use unnumbered footnote in this file to avoid reseting the footnotes numbering.

Example: when building the tutorial into a PDF and using `latex_show_urls = "footnotes"`; this footnote become the number 8. However, without this change, the footnote shows the number 1.
(cherry picked from commit f042646595d1cdd2ecdc15222b1766f746d010f0)

Co-authored-by: Manuel Kaufmann <humitos@gmail.com>