Add clients (game engines) to the matrix

[BastiaanOlij]

This PR adds game engines to our extension support matrix.

Currently only Godot's list of extensions has been completed, other engines added as placeholders.

Note, build on top of #31

[dennyronngren]

I think that the section could change name to "OpenXR adoption matrix" or something similar as it's not just runtimes in it anymore.

I really like having the engines in there. If we get good coverage it would help for our internal runtime planning also.

[fire]

It would be encouraging to have a few more engine vendors to submit their adoption matrix.

[dsnopek]

Are we limiting the list to extensions in stable versions?

If not, we also have XR_KHR_visibility_mask.

However, I could certainly understand only listing extensions supported in stable Godot versions. We could then update this matrix after each stable release.

[BastiaanOlij]

It would be encouraging to have a few more engine vendors to submit their adoption matrix.

That in the end is up to them, same as with the maintenance of the list of runtimes.

I only added them as an example, before this gets merged we need to remove those for which we haven't received updated so the engine vendors can add their own PRs on top.

I have asked for participation and if I get a list of extensions for other engines, I'll be happy to include them in this PR.

[BastiaanOlij]

Are we limiting the list to extensions in stable versions?

If not, we also have XR_KHR_visibility_mask.

However, I could certainly understand only listing extensions supported in stable Godot versions. We could then update this matrix after each stable release.

I think so.

@rpavlik should we add some sort of disclaimer on the page that states there is no guarantee this list is up to date or missing entries?

[BastiaanOlij]

@rpavlik @dennyronngren I've rebased this on master and applied your suggestions both here and given elsewhere.

With the rebase this supersedes Denny's PR.

[BastiaanOlij]

Rebased to include bytedance latest changes.

I don't know enough about the framework we're using for how this is styled but it would seem it would be a good idea to increase the width of the header to match the width of the matrix.