Update Accessibility docs

[hannah-hyj]

This PR did

1. Split Accessibility and Internationalization into two tabs
2. Update Accessibility docs to 5 parts
   • title: Introduction
   • title: UI Design & Styling
   • title: Assistive technologies
   • title: Accessibility testing
   • title: Web accessibility
3. keep old content and add some new content
   • title: Introduction
     added standards and regulation
   • title: UI Design & Styling
     added pictures and touch target size.
   • title: Assistive technologies
     added mobility.
   • title: Accessibility testing
     added nothing.
   • title: Web accessibility
     added how to opt-in using button or code

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 37d151b):

https://flutter-docs-prod--pr12513-refresh-a11y-2dmrp4ce.web.app