Docs: Standardize names and terms for renderers

[tetrapod00]

Part of godotengine/godot-docs#10139.
See also #98747, godotengine/godot-docs#10183.

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:

• "GL Compatibility" to "Compatibility", since that is what shows up in the editor.
• Ambiguous usages of "backend" are made explicit: they are either renderers, rendering drivers, or the RenderingDevice backend.
• Existing references to "rendering methods" are left unchanged, since "rendering method" is still a valid term.

[Mickeon]

Make it a triple approval but the rendering team should really take a look first.

[mhilbrunner]

LGTM, but should be approved by Rendering.

[akien-mga]

While at it, "supports ... available" sounds weird.

Suggested change

	[b]Note:[/b] This only functions on a device which supports either Vulkan, D3D12, or Metal available as a rendering driver.
	[b]Note:[/b] This only functions on a device which supports either Vulkan, D3D12, or Metal as a rendering driver.

[tetrapod00]

Also changed "D3D12" to "Direct3D 12", since it's almost always spelled out.

[Mickeon]

Poor you for just pushing a hour ago. There's a minor merge conflict in rendering/renderer/rendering_method's description.

[akien-mga]

Needs rebase.

[akien-mga]

Thanks!