Rework README and documentation

The README.md page is HUGE. Split it up.

Also document audio forwarding and improve installation instructions for
each platform and user documentation.

PR #3774 <https://github.com/Genymobile/scrcpy/pull/3774>

doc/audio.md

@@ -0,0 +1,90 @@
+# Audio
+
+Audio forwarding is supported for devices with Android 11 or higher, and it is
+enabled by default:
+
+ - For **Android 12 or newer**, it works out-of-the-box.
+ - For **Android 11**, you'll need to ensure that the device screen is unlocked
+   when starting scrcpy. A fake popup will briefly appear to make the system
+   think that the shell app is in the foreground. Without this, audio capture
+   will fail.
+ - For **Android 10 or earlier**, audio cannot be captured and is automatically
+   disabled.
+
+If audio capture fails, then mirroring continues with video only (since audio is
+enabled by default, it is not acceptable to make scrcpy fail if it is not
+available), unless `--require-audio` is set.
+
+
+## No audio
+
+To disable audio:
+
+```
+scrcpy --no-audio
+```
+
+## Codec
+
+The audio codec can be selected. The possible values are `opus` (default), `aac`
+and `raw` (uncompressed PCM 16-bit LE):
+
+```bash
+scrcpy --audio-codec=opus  # default
+scrcpy --audio-codec=aac
+scrcpy --audio-codec=raw
+```
+
+Several encoders may be available on the device. They can be listed by:
+
+```bash
+scrcpy --list-encoders
+```
+
+To select a specific encoder:
+
+```
+scrcpy --audio-codec=opus --audio-encoder='c2.android.opus.encoder'
+```
+
+For advanced usage, to pass arbitrary parameters to the [`MediaFormat`],
+check `--audio-codec-options` in the manpage or in `scrcpy --help`.
+
+[`MediaFormat`]: https://developer.android.com/reference/android/media/MediaFormat
+
+
+## Bit rate
+
+The default video bit-rate is 128Kbps. To change it:
+
+```bash
+scrcpy --audio-bit-rate=64K
+scrcpy --audio-bit-rate=64000  # equivalent
+```
+
+_This parameter does not apply to RAW audio codec (`--audio-codec=raw`)._
+
+
+## Buffering
+
+Audio buffering is unavoidable. It must be kept small enough so that the latency
+is acceptable, but large enough to minimize buffer underrun (causing audio
+glitches).
+
+The default buffer size is set to 50ms. It can be adjusted:
+
+```bash
+scrcpy --audio-buffer=40   # smaller than default
+scrcpy --audio-buffer=100  # higher than default
+```
+
+Note that this option changes the _target_ buffering. It is possible that this
+target buffering might not be reached (on frequent buffer underflow typically).
+
+If you don't interact with the device (to watch a video for example), a higher
+latency (for both [video](video.md#buffering) and audio) might be preferable to
+avoid glitches and smooth the playback:
+
+```
+scrcpy --display-buffer=200 --audio-buffer=200
+```