Rename MP4Box.md to mp4box.md

So it's sorted by default next to all the other lowercase commands

pages/common/mp4box.md

@@ -0,0 +1,16 @@
+# MP4Box
+
+> MPEG-4 Systems Toolbox - Muxes streams into MP4 container
+
+- Display information about an existing MP4 file
+
+`MP4Box -info {{filename}}`
+
+- Add an SRT subtitle file into an MP4 file 
+
+`MP4Box -add {{input-subs.srt}}:lang=eng -add {{input.mp4}} {{output.mp4}}`
+
+- Combine audio from one file and video from another
+
+`MP4Box -add {{input1.mp4}}#audio -add {{input2.mp4}}#video {{output.mp4}`
+