Standardize renderer names and terms

[tetrapod00]

Part of #10139. (Doesn't close, because some changes must be made in the class ref still.)
See also godotengine/godot#98747, godotengine/godot#98744.

Standardizes terms for renderers/rendering methods, rendering drivers, and "rendering backends".

New standards, for user-facing docs:

• Godot has three renderers or rendering methods: Forward+, Mobile, and Compatibility. In most user-facing docs, these are referred to as "renderers". They are exposed by the engine as "rendering methods", and "rendering method" is a valid term to use.
• Godot has four rendering drivers: OpenGL, Vulkan, Direct3D 12, Metal.
• Godot has a rendering abstraction layer called "RenderingDevice". This can be referred to as the "RenderingDevice backend".
• The term "backend" should be avoided in user-facing docs, as it is ambiguous. Especially the terms "Compatibility backend", "Forward+ backend", "Vulkan backend", are to be avoided.
• The Forward+ renderer is called "Forward+". Avoid "Forward Plus" or "forward_plus" unless + is not a valid symbol.
• The Mobile renderer is called "Mobile". Avoid "Forward Mobile". This rendering method is exposed in the project settings as "mobile" and referred to as "Mobile" in the engine interface.
• The Compatibility renderer is called "Compatibility". Only use "GL Compatibility" when needed. For example, gl_compatibility is how it is exposed in the project settings.

This PR changes:

• Almost all references to "Forward Mobile" to "Mobile". I left a lot of existing references on the Internal Rendering Architecture page.
• All references to "Clustered Forward" to "Forward+".
• Most ambiguous "backend" references to explicitly refer to renderers or rendering drivers.
• Some references to "rendering methods" to "renderers". "Rendering method" is still valid terminology.
• Copies the renderer description from the Renderers page to the List of Features page.
• Corrects some incorrect information on the Internal Rendering Architecture page.

Since the existing terms are nonstandardized, I probably missed a few instances.

[skyace65]

Looks good! Just needs a few fixes.

[tetrapod00]

Removed all references to "Forward Mobile" from the manual, except for a single note on the Internal rendering architecture page that says "Forward Mobile" is the internal name.
Removed all references to "Clustered Forward" from the manual.

I believe the original issue and the terminology used was approved by Calinou and Clay, but we should wait to merge this until one of them gives it at least a quick look.

[mhilbrunner]

This needs a rebase, otherwise looks good to me.

[tetrapod00]

Rebased, but this still does need at least a cursory review from someone in rendering

[mhilbrunner]

cc @Calinou and @clayjohn