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/macos.md

@@ -0,0 +1,47 @@
+# On macOS
+
+## Install
+
+Scrcpy is available in [Homebrew]:
+
+```bash
+brew install scrcpy
+```
+
+[Homebrew]: https://brew.sh/
+
+You need `adb`, accessible from your `PATH`. If you don't have it yet:
+
+```bash
+brew install android-platform-tools
+```
+
+Alternatively, Scrcpy is also available in [MacPorts], which sets up `adb` for you:
+
+```bash
+sudo port install scrcpy
+```
+
+[MacPorts]: https://www.macports.org/
+
+_See [build.md](build.md) to build and install the app manually._
+
+
+## Run
+
+Once installed, run from a terminal:
+
+```bash
+scrcpy
+```
+
+or with arguments (here to disable audio and record to `file.mkv`):
+
+```bash
+scrcpy --no-audio --record=file.mkv
+```
+
+Documentation for command line arguments is available:
+ - `man scrcpy`
+ - `scrcpy --help`
+ - on [github](/README.md)