Accessibility: Add documentation around tooling, auditing and a detailed checklist

Created by: umpox

closes https://github.com/sourcegraph/sourcegraph/issues/31238 closes https://github.com/sourcegraph/sourcegraph/issues/31237

Description

This PR adds:

• High level documentation on how to conduct an accessibility audit
• Instructions on how to use a screen reader
• Instructions on how to use ARC Toolkit to find accessibility issues
• Detailed checklist on auditing different parts of a journey for accessibility issues (e.g. tables, forms, images, etc)

Test plan

Documentation. Tested by comparing against Sourcegraph.com and the WCAG guidelines.

How to trial this process

1. Select a small user journey to trial this process on (e.g. Creating a new code monitor)
2. Open How to conduct an accessibility audit
3. Follow the docs. If anything seems confusing/inefficient/wrong - please raise on the PR!