Since joystick support was removed and is a difference from mplayer, it
should be included in the document with the mplayer changes.
It will help new users who were using mplayer's joystick support to
seek alternatives when switching to mpv. It will also be helpful for
people that had problems with the joystick support in mplayer (for
example, by incorrectly recognizing other input devices as joystick)
to know that those problems won't persist in mpv.
Why did this exist in the first place? Other than being completely
useless, this even caused some regressions in the past. For example,
there was the case of a laptop exposing its accelerometer as joystick
device, which led to extremely fun things due to the default mappings of
axis movement being mapped to seeking.
I suppose those who really want to use their joystick to control a media
player (???) can configure it as mouse device or so.
Giving this such a prominent place is not really appropriate anymore.
Most people seeing this would probably expect a release changelog, not
something about MPlayer.
Since the page still could be useful for former MPlayer users (in
particular to avoid confusion with renamed options etc.), still keep
it in the DOCS directory.