Move docs related requirements in the right file

docs-requirements.txt

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

requirements.txt

 arkindex-client==1.0.11
-mkdocs-material==8.5.5
 peewee==3.15.3
 Pillow==9.2.0
 pymdown-extensions==9.6