Pin mkdocs-material to v9.6.9

to fix https://github.com/hello-robot/hello-robot.github.io/actions/runs/15026353263/job/42227966719

Author: hello-binit

requirements.txt

@@ -1,5 +1,5 @@
 mkdocs
-mkdocs-material
+mkdocs-material==9.6.9
 mkdocs-simple-plugin
 pytkdocs[numpy-style]
 jinja2==3.0.3