doc: Sort flatpak-docs includes alphabetically
doc: Sort flatpak-docs includes alphabetically

Sort the list of manpages we're including in flatpak-docs.xml.in, to
make it easier to maintain.

flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak