a11y: Label the code-block copy button

The copy button on code blocks had no accessible name, so screen readers
announced an unlabelled button. Add an aria-label that flips to Copied,
and type=button so it never submits a surrounding form.

Found by the docs site audit in
https://ampcode.com/threads/T-01a0ae50-f7f0-779c-aa66-3887c9953e33

Amp-Thread-ID: https://ampcode.com/threads/T-01a0aed2-5fbe-7768-85ea-1378bf98f9ff
Co-authored-by: Amp <amp@ampcode.com>

Author: marcleblanc2

src/components/CopyButton.tsx

@@ -29,6 +29,8 @@ export function CopyButton({text, lang}: {text: string; lang: string}) {
 				</div>
 				<div className="relative">
 					<button
+						type="button"
+						aria-label={copied ? 'Copied' : 'Copy code'}
 						onClick={handleCopy}
 						onMouseEnter={() => setShowTooltip(true)}
 						onMouseLeave={() => setShowTooltip(false)}