Add documentation for M hotkey.

templates/zerver/help/keyboard-shortcuts.md

@@ -100,3 +100,5 @@ Zulip keyboard shortcuts are divided into four categories:
 * **React to selected message with +1**: `+` - This shortcut allows
   you to react to the selected message (outlined in blue) with the
   +1 (thumbs up) emoji.
+* **Mute/Unmute topic**: `M` - This shortcut toggles the selected
+  message's (outlined in blue) topic between muted and unmuted.

templates/zerver/keyboard_shortcuts.html

@@ -156,6 +156,10 @@
             {% trans %}selected message{% endtrans %}
           </td>
         </tr>
+        <tr>
+          <td class="hotkey">M</td>
+          <td class="definition">{% trans %}Toggle topic mute{% endtrans %}</td>
+        </tr>
         <tr>
           <td class="hotkey">?</td>
           <td class="definition">{% trans %}Show keyboard shortcuts{% endtrans %}</td>