Formalize a11y tools and testing plan

[abbyoung]

Formalize and document tools and development plan to ensure site accessibility.

Some tools have already been set up, and some manual processes won't be necessary until later in the project.

Tools to set up:

• [X] eslint a11y plugin
• [x] cypress-audit (#57)
• [X] storybook accessibility add-on
• [x] jest-axe library (#TBD)

Long-term tasks:

• Manual screenreader testing
• Accessibility audit
• User testing

[abbyoung]

• Created #125
• Compiled resources and tools to use in this document