docs: update calendar, add news item, and link releases notes for 20.0.6

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4822>

docs/index.html

@@ -16,6 +16,8 @@
 
 <h1>News</h1>
 
+<h2>April 29, 2020</h2>
+<p><a href="relnotes/20.0.6.html">Mesa 20.0.6</a> is released. This is a bug fix release.</p>
 <h2>April 22, 2020</h2>
 <p><a href="relnotes/20.0.5.html">Mesa 20.0.5</a> is released. This is a bug fix release.</p>
 <h2>April 3, 2020</h2>

docs/release-calendar.html

@@ -59,13 +59,7 @@ if you'd like to nominate a patch in the next stable release.
 <th>Notes</th>
 </tr>
 <tr>
-<td rowspan="3">20.0</td>
+<td rowspan="2">20.0</td>
-<td>2020-04-29</td>
-<td>20.0.6</td>
-<td>Dylan Baker</td>
-<td></td>
-</tr>
-<tr>
 <td>2020-05-13</td>
 <td>20.0.7</td>
 <td>Dylan Baker</td>

docs/relnotes.html

@@ -21,6 +21,7 @@ The release notes summarize what's new or changed in each Mesa release.
 </p>
 
 <ul>
+<li><a href="relnotes/20.0.6.html">20.0.6 release notes</a></li>
 <li><a href="relnotes/20.0.5.html">20.0.5 release notes</a></li>
 <li><a href="relnotes/20.0.4.html">20.0.4 release notes</a></li>
 <li><a href="relnotes/20.0.3.html">20.0.3 release notes</a></li>