Fix search focus when s/f is pressed

On the Modules matrix page the Search input steals focus when the Module filter input is active and s/f is pressed. This makes it impossible to search for modules containing "s" or "f". It is a known problem in mkdocs-material - it was fixed, however the fix only works for inputs that are within a <form /> (https://github.com/squidfunk/mkdocs-material/issues/398).

This commit wraps the input in a form so that the focus stays in the filter when s/f is pressed.

docs.it4i/modules-matrix.md

 !!! Hint "Cluster Acronyms"
     A - Anselm • S - Salomon • U - uv1 at Salomon
 
-| Module </br><input id="searchInput" placeholder="🔍 Filter" style="width: 8rem; border-radius: 0.2rem; color: black; padding-left: .2rem;"> | Versions | Clusters |
+| Module </br><form><input id="searchInput" placeholder="🔍 Filter" style="width: 8rem; border-radius: 0.2rem; color: black; padding-left: .2rem;"></form> | Versions | Clusters |
 | ------ | -------- | -------- |
 | ABINIT | 7.10.1-foss-2015b</br>7.10.1-intel-2015b</br>8.2.2-intel-2017a</br>8.6.1-intel-2017a</br>8.6.1-intel-2017a-test | `US-`</br>`US-`</br>`-SA`</br>`-S-`</br>`-S-` |
 | absl-py | 0.1.10-Py-3.6 | `-SA` |