[86by8j89h][docs] Add Flags a11y guide and example

[sheila-semrush]

Motivation and Context

1. Added an a11y recommendation that aria-label and role should be used when using Flags without the full country name.
2. Added a corresponding example right to the A11y tab — I think it's not worth complicating this guide's structure while it still doesn't have that much information.

IMPORTANT. This PR already assumes that Flags doesn't have an aria-label, so it should be merged only when the aria-label is actually removed from the component.

How has this been tested?

Manually.

Screenshots:

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [x] Nice improve.

Checklist:

• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly or it's not required.
• [ ] Unit tests are not broken.
• [ ] I have added changelog note to corresponding CHANGELOG.md file with planned publish date.
• [ ] I have added new unit tests on added of fixed functionality.