alsamixer: add page (#16599)

pages/linux/alsamixer.md

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+# alsamixer
+
+> Graphical mixer for ALSA soundcard driver.
+> More information: <https://manned.org/man/alsamixer>.
+
+- Select the soundcard to use:
+
+`alsamixer {{[-c|--card]}} {{soundcard_number}}`
+
+- Select mixer device to control:
+
+`alsamixer {{[-D|--device]}} {{device_name}}`
+
+- Increase volume:
+
+`<w>`
+
+- Decrease volume:
+
+`<x>`
+
+- Quit alsamixer:
+
+`<Esc>`