Document writing shaders that work in multiple renderers

**Your Godot version:**
4.4
**Issue description:**
Since https://github.com/godotengine/godot/pull/97142 allowed falling back from Vulkan/D3D12/Metal to OpenGL, it is now more expected that rendering, including shaders, works on multiple renderers.

We should document writing shaders that work in multiple renderers, using `OUTPUT_SRGB`, `CLIP_SPACE_FAR`, and/or `CURRENT_RENDERER` (once https://github.com/godotengine/godot/pull/98549 is merged).

This is currently half-documented in two places:
- The NDC change from always using OpenGL NDC in 3.x to sometimes using Vulkan NDC in 4.x is documented in https://docs.godotengine.org/en/latest/tutorials/migrating/upgrading_to_godot_4.html#updating-shaders.
- The existence of Vulkan NDC is mentioned in https://docs.godotengine.org/en/latest/tutorials/shaders/advanced_postprocessing.html, but that page does not describe how to write a shader that works with Compatibility which uses OpenGL NDC.

Changes to make:
- Advanced postprocessing should include a snippet for using `CLIP_SPACE_FAR` to create correct NDCs for all renderers.
- Possibly, some shader page should include a general section on writing shaders that work in all renderers. Possibly, the Renderers page should link to that shader section.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/tutorials/migrating/upgrading_to_godot_4.html#updating-shaders
https://docs.godotengine.org/en/latest/tutorials/shaders/advanced_postprocessing.html