Add back-links to advanced topics for Rust and C++

The invalid anchor in the RST is a hack that's required in order to get relative links
working with sphinx (see sphinx-doc/sphinx#701 (comment) )

api/cpp/docs/index.rst

@@ -35,6 +35,15 @@ Slint C++ documentation
 
    genindex
 
+.. toctree::
+   :hidden:
+   :maxdepth: 2
+   :caption: Advanced Topics
+
+   Debugging Techniques <../slint/src/advanced/debugging_techniques.html#http://>
+   Selecting a Widget Style <../slint/src/advanced/style.html#http://>
+   Platform Backends <../slint/src/advanced/backends.html#http://>
+
 .. image:: https://github.com/slint-ui/slint/workflows/CI/badge.svg
    :target: https://github.com/slint-ui/slint/actions
    :alt: GitHub CI Build Status

api/rs/slint/lib.rs

@@ -16,6 +16,9 @@ This crate is the main entry point for embedding user interfaces designed with
 
 */
 #![doc = concat!("* [The Slint Language Documentation](https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint)")]
+#![doc = concat!("* [Debugging Techniques](https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint/src/advanced/debugging_techniques.html)")]
+#![doc = concat!("* [Selecting a Widget Style](https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint/src/advanced/style.html)")]
+#![doc = concat!("* [Platform Backends](https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint/src/advanced/backends.html)")]
 /*! * [Slint on Microcontrollers](docs::mcu)
 
 ## How to use this crate:

docs/reference/src/advanced/backends.md

@@ -1,6 +1,6 @@
 <!-- Copyright © SixtyFPS GmbH <info@slint.dev> ; SPDX-License-Identifier: MIT -->
 
-# Backends
+# Platform Backends
 
 In Slint, a backend is the module that encapsulates the interaction with the operating system,
 in particular the windowing sub-system. Multiple backends can be compiled into Slint and one