Bump Python requirement mkdocs-material to 8.5.6

docs-requirements.txt

 black==22.8.0
 doc8==0.11.1
 mkdocs==1.4.0
-mkdocs-material==8.5.5
+mkdocs-material==8.5.6
 mkdocstrings==0.19.0
 mkdocstrings-python==0.7.1
 recommonmark==0.7.1