Add guidance for mark role (pull #2753)

Updates two practice pages to add information about mark role:
1. On page "Structural Roles", adds mark role to table in section "All Structural Roles and Their HTML Equivalents".
2. On page "Providing Accessible Names and Descriptions", adds mark role to table in section"Accessible Name Guidance by Role".

Fixes issue #1821.

content/practices/names-and-descriptions/names-and-descriptions-practice.html

@@ -929,6 +929,13 @@ <h2><span id="naming_role_guidance_heading">Accessible Name Guidance by Role</sp
                 Using <code>aria-label</code> is functionally equivalent to providing off-screen text in the contents of the log element, except off-screen text would be announced by screen readers that do not support <code>aria-label</code> on <code>log</code> elements.
               </td>
             </tr>
+            <tr>
+              <td>
+                <a href="#mark" class="role-reference"><code>mark</code></a>
+              </td>
+              <td>Prohibited</td>
+              <td></td>
+            </tr>
             <tr>
               <td>
                 <a href="#main" class="role-reference"><code>main</code></a>

content/practices/structural-roles/structural-roles-practice.html

@@ -149,6 +149,10 @@ <h2>All Structural Roles and Their HTML Equivalents</h2>
               <th>listitem</th>
               <td>li</td>
             </tr>
+            <tr>
+              <th>mark</th>
+              <td>mark</td>
+            </tr>
             <tr>
               <th>math</th>
               <td>No equivalent element</td>