No, those are not Geany shortcuts, they are system key sequences and are keyboard specific, as I implied https://github.com/geany/geany/discussions/3887#discussioncomment-9564508 they are not available on common keyboards. These are not Geany specific so they do not belong in a table of Geany shortcuts.
The title of the article at geany.org is "Keyboard shortcuts". In that article there is a section called "Built-in shortcuts". So, my idea is to add at the end of the article a new section called "Keyboard specific shortcuts". The text could be like this: "Some keyboards allow further shortcuts to insert one single character." Then I would add a table with the two shortcuts that are executed with the Alt Gr key. The shortcut "Ctrl-Shift-U, Unicode, Enter" will stay in the "Built-in shortcuts" section.
Is that OK?