[Draft] Add glossary of common terms

[tetrapod00]

Adds a glossary of common terms to the online manual.

Uses the sphinx glossary directive.

On its own, this is perhaps a solution in search of a problem. But it's also relatively low-risk and low-cost to add, since it links to other pages, but no other pages depend on it.

With the addition of links to the glossary, this could be a much more useful improvement. We can link to these terms with the term role. Alternately, we can also integrate hoverxref to add tooltips for common terms like acronyms, which link into the glossary.

See also previous discussion in #1650 (comment).

Kinds of terms that will be included in the glossary:
Very common nodes or godot concepts: control, node, viewport, signal, resource
Godot-specific terms: renderer, rendering driver, server (in the Godot sense)
Godot proper nouns: GDScript, GDExtension, Forward+ renderer, Mobile renderer, Compatibility renderer, RenderingDevice
Other proper nouns: Vulkan, Metal, Direct3D 12
Acronyms, including common gamedev/graphics terms: CPU, GPU, DOF, FXAA, MSAA, SSAA, SSAO, SSIL, SDFGI, HDR, SDR, sRGB

The current definitions are roughly representative of the final page in terms of length. Each term should be briefly defined, with 1-3 sentences, and link to a relevant portion of the documentation. However, the current list of terms is not yet complete.

I'm leaving this as a draft to discuss whether this is something worth including at all, and if so, whether there would need to be major changes to implementation.

The page renders like this with the current theme: