docs: restore sound-mute.svg since used on dashboard
docs: restore sound-mute.svg since used on dashboard

mhenderson-so/bosun,bosun-monitor/bosun,bookingcom/bosun,mhenderson-so/bosun,bosun-monitor/bosun,mhenderson-so/bosun,mhenderson-so/bosun,bosun-monitor/bosun,mhenderson-so/bosun,bookingcom/bosun,bookingcom/bosun,bookingcom/bosun,bosun-monitor/bosun,mhenderson-so/bosun,bookingcom/bosun,bookingcom/bosun,bosun-monitor/bosun,bosun-monitor/bosun