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/window.md

@@ -0,0 +1,55 @@
+# Window
+
+## Title
+
+By default, the window title is the device model. It can be changed:
+
+```bash
+scrcpy --window-title='My device'
+```
+
+## Position and size
+
+The initial window position and size may be specified:
+
+```bash
+scrcpy --window-x=100 --window-y=100 --window-width=800 --window-height=600
+```
+
+## Borderless
+
+To disable window decorations:
+
+```bash
+scrcpy --window-borderless
+```
+
+## Always on top
+
+To keep the window always on top:
+
+```bash
+scrcpy --always-on-top
+```
+
+## Fullscreen
+
+The app may be started directly in fullscreen:
+
+```bash
+scrcpy --fullscreen
+scrcpy -f   # short version
+```
+
+Fullscreen mode can then be toggled dynamically with <kbd>MOD</kbd>+<kbd>f</kbd>
+(see [shortcuts](shortcuts.md)).
+
+
+## Disable screensaver
+
+By default, _scrcpy_ does not prevent the screensaver from running on the
+computer. To disable it:
+
+```bash
+scrcpy --disable-screensaver
+```