docs: add release calendar page and references to it

Add a page that has information which release is expected when and
associated information.

Reference to it from the "Releasing process" and "Release notes" pages.

v2:
 - Add Andres for 17.0.5
 - Rework table format to include the branch (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

docs/relnotes/17.0.4.html

@@ -36,6 +36,13 @@ c4c34ba05d48f76b45bc05bc4b6e9242077f403d63c4f0c355c7b07786de233e  mesa-17.0.4.ta
 </pre>
 
 
+<h2>Next release</h2>
+<p>
+Mesa 17.0.5 is expected in approximatelly two weeks. See the release
+<a href="../release-calendar.html#calendar" target="_parent">calendar</a>
+for details.
+</p>
+
 <h2>New features</h2>
 <p>None</p>
 

docs/relnotes/17.1.0.html

@@ -19,7 +19,8 @@
 <p>
 Mesa 17.1.0 is a new development release.
 People who are concerned with stability and reliability should stick
-with a previous release or wait for Mesa 17.1.1.
+with a previous release or wait for
+<a href="../release-calendar.html#calendar" target="_parent">Mesa 17.1.1</a>.
 </p>
 <p>
 Mesa 17.1.0 implements the OpenGL 4.5 API, but the version reported by