Expand Godot's GLSL documentation

### Describe the project you are working on

Evaluating Godot as a Unity replacement

### Describe the problem or limitation you are having in your project

Hi! I'm a Unity refugee, bear with me here 😅

I had to do quite a bit of digging to figure out that [Godot has a seemingly undocumented ability to read GLSL as vertex and fragment shaders](https://godotengine.org/article/vulkan-progress-report-7/#low-level-access-to-renderingdevice). While this is good news for myself, I found that the current Godot documentation (checked on both `stable` and `latest`) makes no reference to this usage of GLSL, only using [GLSL as compute shaders](https://docs.godotengine.org/en/stable/tutorials/shaders/compute_shaders.html#doc-compute-shaders).

### Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add documentation for this usage of GLSL, allowing shader developers to learn of this otherwise hidden feature of Godot.

### Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Addition of a new section within the Shaders manual (or perhaps an expansion to existing sections) documenting the usage of GLSL as vertex/fragment shaders.

### If this enhancement will not be used often, can it be worked around with a few lines of script?

Less experienced shader developers can simply continue to use Godot's built-in shader language as-is. Otherwise, there exists no official documentation for this usage, so developers will have to do a similar amount of research to find the same Vulkan Progress Report article hinting at this feature.

### Is there a reason why this should be core and not an add-on in the asset library?

This is a currently undocumented feature within the latest stable release of Godot, and should be documented like the rest of the engine.