opusenc: move from linux to common (#4860)

pages/common/opusenc.md

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+# opusenc
+
+> Convert WAV or FLAC audio to Opus.
+> More information: <https://opus-codec.org/docs/opus-tools/opusenc.html>.
+
+- Convert WAV to Opus using default options:
+
+`opusenc {{path/to/input.wav}} {{path/to/output}}.opus`
+
+- Convert stereo audio at the highest quality level:
+
+`opusenc --bitrate {{512}} {{path/to/input.wav}} {{path/to/output}}.opus`
+
+- Convert 5.1 surround sound audio at the highest quality level:
+
+`opusenc --bitrate {{1536}} {{path/to/input.flac}} {{path/to/output}}.opus`
+
+- Convert speech audio at the lowest quality level:
+
+`opusenc {{path/to/input.wav}} --downmix-mono --bitrate {{6}} {{path/to/out}}.opus`