Improve docs about backend and render selection

CHANGELOG.md

@@ -7,6 +7,7 @@ All notable changes to this project are documented in this file.
 ## General
 
  - The minimum Rust version is now 1.73.
+ - When the Skia renderer is enabled at compile time, it is picked as the default at run-time.
  - FemtoVG renderer: Fixed selection of italic font styles. (#5056)
  - Fixed color animation involving transparency. (#5063)
  - Android: Fixed support for Android 10 to 12 and Java 1.8.

docs/reference/src/advanced/backend_qt.md

@@ -10,6 +10,12 @@ macOS, Windows, Linux with Wayland and X11, and direct full-screen rendering via
 The Qt backend only supports software rendering at the moment. That means it runs with any graphics driver,
 but it does not utilize GPU hardware acceleration.
 
+The compilation step will detect whether Qt is installed or not using the qttype crate.
+See the instructions in the [qttypes documentation](https://docs.rs/qttypes/latest/qttypes/#finding-qt)
+on how to set environment variables to point to the Qt installation.
+
+If Qt is not installed, the backend will be disabled, and Slint will fallback to another backend, usually the [Winit backend](backend_winit.md).
+
 ## Configuration Options
 
 The Qt backend reads and interprets the following environment variables:

docs/reference/src/advanced/backend_winit.md

@@ -18,6 +18,9 @@ The Winit backend supports different renderers. They can be explicitly selected
 | Skia OpenGL   | OpenGL rendering with Skia                  | `winit-skia-opengl`                      |
 | software      | Software-rendering, no GPU required         | `winit-software`                         |
 
+If no renderer is explicitly set, the backend will first try to use the Skia renderer, if it was enabled at compile time.
+If that fails, it will fall back to the FemtoVG renderer, and if that also fails, it will use the software renderer.
+
 
 ## Configuration Options