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/linux.md

@@ -0,0 +1,79 @@
+# On Linux
+
+## Install
+
+<a href="https://repology.org/project/scrcpy/versions"><img src="https://repology.org/badge/vertical-allrepos/scrcpy.svg" alt="Packaging status" align="right"></a>
+
+Scrcpy is packaged in several distributions and package managers:
+
+ - Debian/Ubuntu: `apt install scrcpy`
+ - Arch Linux: `pacman -S scrcpy`
+ - Fedora: `dnf copr enable zeno/scrcpy && dnf install scrcpy`
+ - Gentoo: [ebuild][ebuild-link] file
+ - Snap: `snap install scrcpy`
+ - … (see [repology](https://repology.org/project/scrcpy/versions))
+
+[ebuild-link]: https://github.com/maggu2810/maggu2810-overlay/tree/master/app-mobilephone/scrcpy
+
+### Latest version
+
+However, the packaged version is not always the latest release. To install the
+latest release from `master`, follow this simplified process.
+
+First, you need to install the required packages:
+
+```bash
+# for Debian/Ubuntu
+sudo apt install ffmpeg libsdl2-2.0-0 adb wget \
+                 gcc git pkg-config meson ninja-build libsdl2-dev \
+                 libavcodec-dev libavdevice-dev libavformat-dev libavutil-dev \
+                 libswresample-dev libusb-1.0-0 libusb-1.0-0-dev
+```
+
+Then clone the repo and execute the installation script
+([source](/install_release.sh)):
+
+```bash
+git clone https://github.com/Genymobile/scrcpy
+cd scrcpy
+./install_release.sh
+```
+
+When a new release is out, update the repo and reinstall:
+
+```bash
+git pull
+./install_release.sh
+```
+
+To uninstall:
+
+```bash
+sudo ninja -Cbuild-auto uninstall
+```
+
+_Note that this simplified process only works for released versions (it
+downloads a prebuilt server binary), so for example you can't use it for testing
+the development branch (`dev`)._
+
+_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)