feat: add accessibility Github Actions workflow

[hbuchel]

Description of changes:

Example run with violations:
https://github.com/aws-amplify/docs/actions/runs/9276059622/job/25522208433?pr=7669

Related GitHub issue #, if available:

Instructions

If this PR should not be merged upon approval for any reason, please submit as a DRAFT

Which product(s) are affected by this PR (if applicable)?

• amplify-cli
• amplify-ui
• amplify-studio
• amplify-hosting
• amplify-libraries

Which platform(s) are affected by this PR (if applicable)?

• JS
• Swift
• Android
• Flutter
• React Native

Please add the product(s)/platform(s) affected to the PR title

Checks

• Does this PR conform to the styleguide?

• Does this PR include filetypes other than markdown or images? Please add or update unit tests accordingly.

• Are any files being deleted with this PR? If so, have the needed redirects been created?

• Are all links in MDX files using the MDX link syntax rather than HTML link syntax?
  
  ref: MDX: [link](https://docs.amplify.aws/)
  HTML: <a href="https://docs.amplify.aws/">link</a>

When this PR is ready to merge, please check the box below

• Ready to merge

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[timngyn]

not blocking: Could we use async/await here? I know one of our utilities function started using await, but I know we still have other scripts using the promises chaining, so this comment isn't blocking

example of async and await from one of our scripts:

docs/.github/workflows/scripts/utilities.js

Lines 14 to 22 in 19117c7

	const deletedFiles = (
	await github.paginate(
	'GET /repos/{owner}/{repo}/pulls/{pull_number}/files',
	{ owner, repo, pull_number: issue_number },
	(response) => response.data
	)
	)
	.filter((file) => file.status === 'removed')
	.filter((file) => paths.some((path) => file.filename.startsWith(path)));

[hbuchel]

I updated this in latest commit to use async/await instead (and some formatting/comment updates)

[timngyn]

This is awesome, great job @hbuchel!!