Docs: add Zenodo link th RTD

Author: MuellerSeb

docs/source/_templates/layout.html

@@ -5,6 +5,7 @@
     <br />
     <p class="link caption"><span class="link caption-text">PyKrige Links</span></p>
     <a href="https://github.com/GeoStat-Framework/PyKrige">PyKrige GitHub</a>
+    <a href="https://doi.org/10.5281/zenodo.3738604">PyKrige Zenodo DOI</a>
     <a href="https://pypi.org/project/PyKrige/">PyKrige PyPI</a>
     <br />
     <p class="link caption"><span class="link caption-text">GeoStat Framework</span></p>