Previously, there were three different DTDs used. Let's switch to a single one.
We will go with 4.5, since it is latest version that does not have any backwards incompatible changes.
Add a flatpak-spawn(1) man page. While it is not ideal
to have the docs live separately from the code, having
it here ensures that it will be available everywhere
all the other flatpak documentation is.
Closes: #2437
Approved by: alexlarsson
