[Document Design System] Write component/pattern accessibility docs

[bethcollins92]

From https://github.com/canonical-web-and-design/vanilla-squad/issues/1100:

for Dev, to add accessibility content in each component's implementation spec, including

• "How it works" explaining why aria attributes etc are important for accessibility
• "accessibility consideration" about how a developer should consider accessibility when using this component
• "Resources" (or reference) from W3C, WCAG and WebAIM, etc

[bartaz]

We should move this to JIRA.

Child issues (for better visibility outside zenhub):

[Accessibility Docs] Slider vanilla-framework #4063 [Accessibility Docs] List tree vanilla-framework #4050 [Accessibility Docs] Matrix vanilla-framework #4053 [Accessibility Docs] Media object vanilla-framework #4054 [Accessibility Docs] Muted heading vanilla-framework #4056 [Accessibility Docs] Search and filter vanilla-framework #4061 [Accessibility Docs] Table of contents vanilla-framework #4066