Merge Sections on Post-Processing/Screen-Reading Shaders

**Your Godot version:**
4.0.3

**Issue description:**
The latest version of the Godot manual has three different sections on implementing post-processing shaders (linked below). Each of them introduces a different aspect (back-buffer copies, full-screen quad, full-screen canvas layer) and its benefits/drawbacks, but they never draw a full picture due to being partially outdated (e.g. the section "Screen-reading shaders" still suggests creating Viewports, which is not possible anymore), making it necessary to cross-reference a lot to understand a single one of them. This also makes it hard to tell which approach is most suitable for which context.

On top of that, two of the sections mentioned are named "Custom Post-Processing" and "Advanced Post-Processing". Personally I find this confusing, as those titles are almost synonymous despite describing two very different things.

I therefore propose that all three sections shall be merged into one that explains and compares the approaches in a more systematic way.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/tutorials/shaders/screen-reading_shaders.html
https://docs.godotengine.org/en/latest/tutorials/shaders/advanced_postprocessing.html
https://docs.godotengine.org/en/latest/tutorials/shaders/custom_postprocessing.html