This reverts commit 398c048c66.
It got removed because it used to cause build issues, but now that we
have a CI let's hope it won't be a problem any more.
This example has been causing on-and-off build breaks for quite some
time. In this case, the code for copying the generated content into the
main docs of GIO is causing problems with srcdir != destdir builds (due
to the files also being copied from the read-only srcdir during
distchecks).
We could probably work around this problem yet again, but since there is
no real benefit to having this content included, so let's remove it.
https://bugzilla.gnome.org/show_bug.cgi?id=734469