Add WCAG and ARIA intro

[steff456]

This PR is part of #125

It revamps the useful resources by adding a section with an intro to WAI-ARIA and WCAG guidelines.

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📚 Documentation preview 📚: https://jupyter-accessibility--126.org.readthedocs.build/en/126/

[isabela-pf]

Thank you for updating this document @steff456! It was much needed. I've mostly reviewed for accuracy or adding information where we've learned the hard way that we need to be careful (like the ARIA section).

I tried to add as many comments as possible as suggestions for clarity, but please feel free to take the review and make changes without the suggestions if you see another way you'd like to integrate it into the document. There are good bones here, and it made it easy for me to review.

I do not know how to change the title of this document, but I think Useful Jupyter Accessibility Resources might be worth updating since the things listed here are no longer Jupyter-specific.

Let me know if you have any questions!