meson: only install docs/man pages when they exist (release tarball)
meson: only install docs/man pages when they exist (release tarball)

related to #4086

Airblader/i3-original,Airblader/i3,i3/i3,stapelberg/i3,Airblader/i3,Airblader/i3,i3/i3,i3/i3,stapelberg/i3,stapelberg/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3,stapelberg/i3,i3/i3,Airblader/i3-original,Airblader/i3-original