Add Mattercraft section to getting-started

[nyan-left]

Hi there,

I've put together a brief getting-started section on Mattercraft. It includes a description, key features, screenshots, and links to docs/tutorials.

Could we merge this into the repo to help developers get started with building WebXR experiences using Mattercraft?
#99

Thanks for considering!

[nyan-left]

Hey @AdaRoseCannon,

Just wanted to give this PR a gentle bump. Opened it a couple weeks ago. Would be great if you could take a look when you have a chance and let me know if you have any feedback.

Cheers!