Easier to understand Qt6 shader documentation

Currently, Qt users get confused and can't find the proper documentation on how to use, compile and edit shaders with Qt6.

The information is spread in multiple non-linked pages or in blog posts. A lack of examples also make it difficult to grasp the necessary steps.

The suggestions around documentation are:

• review the existing pages regarding readability, correctness and discoverability.
• create an "Shader overview" page, to be a supporting helping page where users can understand high level concepts and find the links for more in depth materials
• Add information on how to write and compile shaders with Qt6
• Add updated or new examples on how to achieved what is described on the documentation

Links to pages where information can be found:

• https://doc.qt.io/qt-6/qml-qtquick-shadereffect.html
• https://doc.qt.io/qt-6/qsgmaterialshader.html
• https://doc.qt.io/qt-6/qtshadertools-overview.html
• https://doc.qt.io/qt-6/qtshadertools-build.html
• https://doc.qt.io/qt-6/qtshadertools-index.html
• https://doc.qt.io/qt-6/qtquick3d-custom.html#our-first-vertex-shader
• https://www.qt.io/blog/in-depth-custom-shader-effects