Draft: enable search bar when updating mkdocs-material to ver 9.x

changed line 72 in ./doc.zih.tu-dresden.de/tud_theme/partials/header.html
from
{% if "search" in config["plugins"] %}
to
{% if "material/search" in config["plugins"] %}

solution is described here:
https://squidfunk.github.io/mkdocs-material/upgrade/#changes-to-html-files
and
https://github.com/squidfunk/mkdocs-material/issues/4581

requested review from @mflehmig--tu-dresden.de

assigned to @notr584d--tu-dresden.de

related to !800 (merged) and issue #460 (closed)