docs: update calendar, add news item, and link releases notes for 20.0.4

Note that the next 20.0.x releases numbers have been shifted as this was
not one of the planned releases.

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4428>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4428>

docs/index.html

@@ -16,6 +16,8 @@
 
 <h1>News</h1>
 
+<h2>April 3, 2020</h2>
+<p><a href="relnotes/20.0.4.html">Mesa 20.0.4</a> is released. This is an emergency release which reverts a serious SPIR-V regression in the 20.0.3 release. Users of 20.0.3 are recommended to upgrade immediately.</p>
 <h2>April 1, 2020</h2>
 <p><a href="relnotes/20.0.3.html">Mesa 20.0.3</a> is released. This is a bug fix release.</p>
 <h2>March 18, 2020</h2>

docs/release-calendar.html

@@ -61,26 +61,26 @@ if you'd like to nominate a patch in the next stable release.
 <tr>
 <td rowspan="4">20.0</td>
 <td>2020-04-15</td>
-<td>20.0.4</td>
-<td>Dylan Baker</td>
-<td></td>
-</tr>
-<tr>
-<td>2020-04-29</td>
 <td>20.0.5</td>
 <td>Dylan Baker</td>
 <td></td>
 </tr>
 <tr>
-<td>2020-05-13</td>
+<td>2020-04-29</td>
 <td>20.0.6</td>
 <td>Dylan Baker</td>
 <td></td>
 </tr>
 <tr>
-<td>2020-05-27</td>
+<td>2020-05-13</td>
 <td>20.0.7</td>
 <td>Dylan Baker</td>
+<td></td>
+</tr>
+<tr>
+<td>2020-05-27</td>
+<td>20.0.8</td>
+<td>Dylan Baker</td>
 <td>Last planned release of the 20.0 series</td>
 </tr>
 <tr>

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/20.0.4.html">20.0.4 release notes</a></li>
 <li><a href="relnotes/20.0.3.html">20.0.3 release notes</a></li>
 <li><a href="relnotes/20.0.2.html">20.0.2 release notes</a></li>
 <li><a href="relnotes/19.3.5.html">19.3.5 release notes</a></li>