General accessibility docs

[clairesunstudio]

https://docs.google.com/document/d/15jj3Egw1_ebxDg8KR6OPD_ayHZcSzHhJgSvTZjyTbnM/edit

What is web accessibility?
Why is it important?
Standards:
Compliance level: WCAG 2.0 + 2.1 (AA → AAA)
Why? Section 508 and inclusive experience
High level elements: color, font, markup forms, images, document, structured data, alt text, etc.
One on one section → for developer/designers/content authors
Tools and resources
Testing tools
Screen reader simulator
Color contrast checker
Testing process

[ygannett]

docs-accessibility is pushed with the first edit.
All existing content is kept below the new content.