docs: update calendar, add news item and link release notes for X.Y.Z

Signed-off-by: Dylan Baker <dylan@pnwbakers.com>

docs/index.html

@@ -15,6 +15,12 @@
 <div class="content">
 
 <h1>News</h1>
+<h2>September 7, 2018</h2>
+<p>
+<a href="relnotes/18.1.8.html">Mesa 18.1.8</a> is released.
+This is a bug-fix release.
+</p>
+
 <h2>August 24, 2018</h2>
 <p>
 <a href="relnotes/18.1.7.html">Mesa 18.1.7</a> is released.

docs/release-calendar.html

@@ -39,13 +39,7 @@ if you'd like to nominate a patch in the next stable release.
 <th>Notes</th>
 </tr>
 <tr>
-<td rowspan="2">18.1</td>
-<td>2018-09-07</td>
-<td>18.1.8</td>
-<td>Dylan Baker</td>
-<td></td>
-</tr>
-<tr>
+<td rowspan="1">18.1</td>
 <td>2018-09-21</td>
 <td>18.1.9</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/18.1.8.html">18.1.8 release notes</a>
 <li><a href="relnotes/18.1.7.html">18.1.7 release notes</a>
 <li><a href="relnotes/18.1.6.html">18.1.6 release notes</a>
 <li><a href="relnotes/18.1.5.html">18.1.5 release notes</a>