magick-mogrify: add gif optimization (#14854)

Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>

pages/common/magick-mogrify.md

@@ -24,3 +24,7 @@
 - Double the brightness of all image files in the current directory:
 
 `magick mogrify -modulate {{200}} {{*}}`
+
+- Reduce file sizes of all GIF images in the current directory by reducing quality:
+
+`magick mogrify -layers 'optimize' -fuzz {{7%}} {{*.gif}}`