docs: update calendar, add news item and link release notes for 18.3.6

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

docs/index.html

@@ -15,6 +15,16 @@
 <div class="content">
 
 <h1>News</h1>
+<h2>April 5, 2019</h2>
+<p>
+<a href="relnotes/18.3.6.html">Mesa 18.3.6</a> is released.
+This is a bug-fix release.
+<br>
+NOTE: It is anticipated that 18.3.6 will be the final release in the
+18.3 series. Users of 18.3 are encouraged to migrate to the 19.0
+series in order to obtain future fixes.
+</p>
+
 <h2>March 27, 2019</h2>
 <p>
 <a href="relnotes/19.0.1.html">Mesa 19.0.1</a> is released.

docs/release-calendar.html

@@ -49,13 +49,6 @@ if you'd like to nominate a patch in the next stable release.
 <th>Notes</th>
 </tr>
 <tr>
-<td rowspan="1">18.3</td>
-<td>2019-03-27</td>
-<td>18.3.6</td>
-<td>Emil Velikov</td>
-<td>Last planned 18.3.x release</td>
-</tr>
-<tr>
 <td rowspan="5">19.0</td>
 <td>2019-04-09</td>
 <td>19.0.2</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/18.3.6.html">18.3.6 release notes</a>
 <li><a href="relnotes/19.0.1.html">19.0.1 release notes</a>
 <li><a href="relnotes/18.3.5.html">18.3.5 release notes</a>
 <li><a href="relnotes/19.0.0.html">19.0.0 release notes</a>