chore: add a11y step to ci for wcag aa guidelines

[SethFalco]

Adds a new step to CI to enforce accessibility guidelines.

For now, I'm only going for AA, and I am ignoring the following errors:

• Prism code blocks.
• Something that I'm convinced is an error on the theme rather than our styles.

Changes

• When optimizing an SVG in the playground, I changed the shade of green so that it's actually legible on light mode. Turns out before it was hard to read.
• Fixed the keyboard shortcut colors in light mode as well. It was very difficult to read.

(Probably obvious that I'm not a big light mode fan.)

Chores

I also did some other chores just to align the project with conventions I apply across my other projects.

• Renamed main.yml to ci.yml.
• Omitted names from actions/jobs, I prefer to just see the file name/ID.
• Updated actions and Node.js version.