Link to demos from within the class reference

[aaronfranke]

This was briefly discussed on IRC awhile back and reduz thinks it's a good idea. There is probably a lot more that can be linked, but this work is really monotonous and takes a long time, so I'm burned out for now and I'm submitting what I have so far.

I was wondering what the criteria should be for linking to a demo, since many classes have more than one demo, and we don't have (and can't realistically have) a demo for each individual feature. For this PR, I decided on a generic general rule of "if the demo helps users understand the feature, link it".

There is also some linking of non-demo things in this PR, such as linking more documentation pages for vectors.

[akien-mga]

Given the amount of work done here I don't want to nitpick about casing, but we might need to define a guideline for what case style to use for link titles.

Current examples from existing and added titles:

• Advanced vector math (Sentence case)
• 2D Sprite animation (Sentence case, but note Sprite is the Node name here, not the English noun - though it's debatable whether it shouldn't be the noun, especially since it's renamed Sprite2D in 4.0)
• 2D Dodge The Creeps Demo (Start Case, or invalid use of AP/Chicago style Title Case, as "the" should be lowercase)
• GUI in 3D Demo (Title Case)

IMO that can be fixed in a follow-up PR reviewing the output of rg -g'*.xml' "link title=", after we've agreed on what style should be used consistently.

As always I'm strongly biased towards using Sentence case / against Title Case, which is the "consensus" that I somewhat imposed on godot-docs :)
But at the same time Juan's use of Title Case in the Godot editor labels has also been elevated to consensual, so it's not Manichean.

[akien-mga]

I'll merge as is anyway as it's a huge amount of work and already good as is. As mentioned above we can refine titles further once there's a consensus on style.

[akien-mga]

Thanks!

[aaronfranke]

@akien-mga For the demo links I used the same names as on the asset library, and for the documentation links I used the same names as the documentation articles. At some point we could have a discussion on style.

[akien-mga]

Cherry-picked for 3.2.4.