Add documentation for hashing passwords

Fixes #7432

Adds HTTP API documentation as well as `rabbitmqctl hash_password` docs.

Author: lukebakken

deps/rabbitmq_management/priv/www/api/index.html

@@ -1157,6 +1157,16 @@ <h2>Reference</h2>
           A list of authentication attempts by remote address and username.
         </td>
       </tr>
+      <tr>
+        <td>X</td>
+        <td></td>
+        <td></td>
+        <td></td>
+        <td class="path">/api/auth/hash_password/<i>plaintext-password</i></td>
+        <td>
+          Hashes <code>plaintext-password</code> according to the currently configured password hashing algorithm.
+        </td>
+      </tr>
     </table>