Add image enlarge

Author: jlenh

doc/how-to/how-to-make-a-release.rst

@@ -27,6 +27,7 @@ during the testing phase of the release candidate, make as many release
 candidates for ESMValCore as needed in order to fix them.
 
 .. figure::  ../figures/release-timeline-doodle-esmvalcore.png
+   :target: ../figures/release-timeline-doodle-esmvalcore.png
    :align:   center
 
    Example of a release timeline for ESMValCore (in this case for 2.11.0)