Docs: Add avol(1)

Base/usr/share/man/man1/avol.md

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+## Name
+
+avol - Change system sound volume
+
+## Synopsis
+
+```**sh
+$ avol [-m] [-M] [volume]
+```
+
+## Description
+
+This program is used to control the AudioServer volume and muted state.
+
+## Options
+
+* `-m`: Mute all audio.
+* `-M`: Unmute all audio.
+
+## Examples
+
+```sh
+# Set the volume to 100
+$ avol 100
+Volume: 100
+
+# Get the current volume
+$ avol
+Volume: 80
+
+# Mute all audio
+$ avol -m
+
+# Unmute all audio
+$ avol -M
+```