feat(Accessibility): Initial addition of accessibility audit guides

To enhance our section on Accessibility, I’m adding a sub-section with guides for performing accessibility audits. These are high-level guides that don’t even get into WCAG SC. The goal is awareness and empathy, while learning some entry-level ways to audit websites with different modalities in mind.

This was not added to the handbook because it relates to the Accessibility section in Pajamas.

This initial MR includes an overview and the first section. Placeholders are included for subsequent sections that are in the works.