Add ATK and AX API mapping for <mphantom>

See discussion in [1]. Per MathML Core [2], `<mphantom>` has style
`visibility: hidden` by default and so is not exposed to ATs [3].
This commit specifies how to map `<mphantom>` when the element is
exposed (e.g. by reverting `visibility: hidden`), essentially
following what exists for the `<mrow>` element.

[1] w3c/mathml-aam#9
[2] https://w3c.github.io/mathml-core/#making-sub-expressions-invisible-mphantom
[3] https://www.w3.org/TR/wai-aria-1.2/#tree_exclusion

Author: fred-wang

mathml-aam/index.html

@@ -382,6 +382,20 @@ <h3>MathML Element Mappings</h3>
 		    <span class="subrole">AXSubrole: <code>TBD</code></span>
 		  </td>
 		</tr>
+		<tr id="el-phantom">
+		  <th><a data-cite="MathML3/chapter3.html#presm.mphantom">`mphantom`</a></th>
+		  <td class="aria">No corresponding role</td>
+		  <td class="ia2">TBD</td>
+		  <td class="uia">TBD</td>
+		  <td class="atk">
+		    <span class="role">Role: <code>ATK_ROLE_SECTION</code></span><br />
+		    <span class="objattr">Object Attribute: <code>tag:mphantom</code></span>
+		  </td>
+		  <td class="axapi">
+		    <span class="role">AXRole: <code>NSAccessibilityGroupRole</code></span><br />
+		    <span class="subrole">AXSubrole: <code>AXMathRow</code></span>
+		  </td>
+		</tr>
 		<tr id="el-mprescripts">
 		  <th><a data-cite="MathML3/chapter3.html#presm.mmultiscripts">`mprescripts`</a></th>
 		  <td class="aria">No corresponding role</td>