[desktop]: Improve the explanations on how to add and remove audio and video filter

Currently: https://docs.videolan.me/vlc-user/desktop/3.0/en/basic/settings/adjustmentsandeffects.html#video-effects

The page doesn't explain the impact on performances and showing how to reach this panel could be improved.

See post: https://forum.videolan.org/viewtopic.php?p=542575#p542575