Repurpose repo

[NellWaliczek]

This repo was cloned from the original WebXR repo, and this PR removes a bunch of text and images unrelated to Augmented Reality features in WebXR. This includes:

• Removing and altering the text in the index.bs file
• Removing input-explainer.md, spatial-tracking-explainer.md and associated images
• Removing non-AR text from the explainer.md which is now the ar-module-explainer.md
• Removing all the design docs
• Updating the README.md

This is by no means intended to be creating a complete/polished spec or explainer; this module is just getting started. As such, we may end up deciding that some of the content would be better over in a different, more incubation focused repo. This key thing is that this PR establishes a jumping off point to ensure that the content removed from the webxr repo doesn't get lost.

[NellWaliczek]

I am merging this PR based on the resolution in today's call, and the fact that this PR has been open a while with an approval.

A subsequent PR to be posted shortly will fix up the environment blend mode as decided on today's call.