docs: add f-theta lens with fold mirror to specialized lenses gallery

Author: HarrisonKramer

docs/gallery/specialized_lenses.rst

@@ -9,6 +9,7 @@ While these lenses are perhaps more complex, they are built using the same basic
     specialized_lenses/double_Gauss
     specialized_lenses/objective
     specialized_lenses/f_theta_lens
+    specialized_lenses/f_theta_with_fold_mirror
     specialized_lenses/microscope
     specialized_lenses/catadioptric
     specialized_lenses/endoscope