feat: update doc

Author: hacdias

core/corehttp/gateway_handler_codec.go

@@ -127,9 +127,9 @@ func (i *gatewayHandler) serverCodecHTML(ctx context.Context, w http.ResponseWri
 			<pre>%s</pre>
 			<p>Please follow the following links to download the document in other formats:</p>
 			<ul>
-				<li><a href="?format=dag-json">DAG-JSON</a></li>
-				<li><a href="?format=dag-cbor">DAG-CBOR</a></li>
-				<li><a href="?format=raw">Raw</a></li>
+				<li><a href="?format=dag-json">Download in DAG-JSON</a> (<a href="https://ipld.io/specs/codecs/dag-json/spec/">format specification</a>)</li>
+				<li><a href="?format=dag-cbor">Download in DAG-CBOR</a> (<a href="https://ipld.io/specs/codecs/dag-cbor/spec/">format specification</a>)</li>
+				<li><a href="?format=raw">Download Raw Block</a></li>
 			</ul>
 		</body>
 	</html>