[DOC] - Add a11y documentation to the JupyterLab's contributor docs

[trallard]

Background

Improve Jupyterlab's contributor documentation to account for accessibility testing and best practices as part of the workflow.

Expected timeline

We can also document as we do work on our Team Compass
Should be done by the 18 months milestone

Identified content

Beta Give feedback

1. WCAG intro and links to resources - @isabela-pf @steff456
2. What to look for when reviewing PRs 📌 - design @isabela-pf (mentioned on jupyter/accessibility #125 comment)
3. Write up results from Rendered notebook user testing - Round 1 #146 and Rendered notebook user testing - Round 2 #147 in iota-school/notebooks-for-all. Link to user testing in jupyter/accessibility and jupyter/surveys
4. Write up results from JupyterLab user testing for common workflows #184 in quansight-labs/jupyterlab-user-testing and/or jupyter/accessibility. Link to user testing in jupyter/accessibility and jupyter/surveys

Future work

Beta Give feedback

1. Later based on our automated actions - how to use the automated tests
2. Best practices -> use colour variables - Add accessibility-focused documentation to the developer documetation jupyterlab/jupyterlab#14399