Accessibility basics documentation

[jessegreenberg]

PhET has been working on a quick start guide for "Tier 1" for accessibility features. It would be great to move that content into SceneryStack documentation. "Tier 1" is likely to be renamed. But it includes a set of basic accessibility features that significantly improve accessibility and would make a nice intro to accessibility or "accessibility basics".

I am going to check in some documentation to a branch and from there we can then discuss how to best integrate it into SceneryStack.

[jessegreenberg]

Pushed to https://github.com/scenerystack/community/tree/accessibility-basics.

Main file is scenery-accessibility-basics.md which can be found under "guides".