Resolve TODO in sorting docs: add accessibility guidelines

What does this MR do?

Resolves the TODO placeholder in the Sorting component documentation's accessibility section.

Adds guidelines covering:

• Labeling the sort dropdown for screen readers (visible label or aria-label)
• Accessible name for the sort direction toggle button (sortDirectionToolTip prop)
• Behavior when the direction toggle is disabled for certain sort options (aria-disabled with explanatory name)
• Live region announcement after sort changes

Part of the TODO resolution effort tracked in #3521.

Screenshots or screen recordings

N/A — text-only changes.

Integrations

N/A

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Conformity

• The "What does this MR do?" section in the MR description is filled out.
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• The MR is added to a milestone.
• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update.

Documentation

• Content follows the GitLab Documentation Style Guide when appropriate.