build: fix for `make distcheck` without enable-gtk-doc

build: fix for `make distcheck` without enable-gtk-doc

Without this patch, the following fails with a rather obscure message
about missing make target.

  ./autogen.sh && make && make distcheck
  ...
  *** No rule to make target `NetworkManager.8', needed by `distdir'. Stop.

Swap the order of the subdirectories 'docs' and 'man' to build
'docs' earlier. This way, `make distcheck` fails in the directory 'docs'
with a better error message:
  *** gtk-doc must be installed and enabled in order to make dist

Also, add 'man/nmcli-examples.5' to the list of files, to determine
whether to use the pre generated doc files.

Signed-off-by: Thomas Haller <thaller@redhat.com>