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>
diff --git a/docs/contents.html b/docs/contents.html
index 90a1a00..d545542 100644
--- a/docs/contents.html
+++ b/docs/contents.html
@@ -84,6 +84,7 @@
 <li><a href="codingstyle.html" target="_parent">Coding Style</a>
 <li><a href="submittingpatches.html" target="_parent">Submitting patches</a>
 <li><a href="releasing.html" target="_parent">Releasing process</a>
+<li><a href="release-calendar.html" target="_parent">Release calendar</a>
 <li><a href="sourcedocs.html" target="_parent">Source Documentation</a>
 <li><a href="dispatch.html" target="_parent">GL Dispatch</a>
 </ul>