docs: add acknowledgement for M3ODEL funding (#1675)

Author: slayoo

docs/templates/index.html.jinja2

@@ -177,7 +177,8 @@ jupyter-notebook PySDM/examples</code></pre>
             <li><a href="https://science.energy.gov">US Department of Energy</a> via the <a href="https://www.krellinst.org/csgf/">Computational Science Graduate Fellowship</a> and via the <a href="https://asr.science.energy.gov/">Atmospheric System Research Program</a> (grant no. DE-SC0021034)</li>
             <li><a href="https://www.schmidtfutures.org/our-people/#eric-schmidt">Eric</a> and <a href="https://www.schmidtfutures.org/our-people/#wendy-schmidt">Wendy Schmidt</a> (by recommendation of <a href="https://www.schmidtfutures.org/">Schmidt Futures</a>)</li>
             <li><a href="https://www.hsfoundation.org/">Heising-Simons Foundation</a></li>
-            <li><a href="https://www.nsf.gov/">National Science Foundation</a> (grant AGS-1835860)</li>
+            <li><a href="https://www.nsf.gov/">US National Science Foundation</a> (grant AGS-1835860)</li>
+            <li><a href="https://model.uni-mainz.de/">Mainz Institute of Multiscale Modeling</a> (German State of Rhineland-Palatinate funding via the <a href="https://model.uni-mainz.de/junior-fellowship/">M3ODEL Junior Fellowship</a>)</li>
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