Add sys-mor-intro to index.md

index.html

 <h1 id="school-material">school-material</h1>
 <ul>
 <li><a href="rb-intro/">RB exercises</a></li>
+<li><a href="sys-mor-intro/">System-theoretic MOR exercises</a></li>
 </ul>

index.md

 # school-material
 
 - [RB exercises](rb-intro/)
-
+- [System-theoretic MOR exercises](sys-mor-intro/)