docs: remove more unused extensions

(cherry picked from commit 4dbf97e3)

docs/requirements.txt

@@ -5,7 +5,6 @@ mkdocs
 mkdocs-material-igwn
 
 # extensions
-markdown-include
 pymdown-extensions >=7.0
 
 # scripts

mkdocs.yml

@@ -20,9 +20,7 @@ theme:
 markdown_extensions:
   - admonition
   - attr_list
-  - markdown.extensions.codehilite
-  - markdown_include.include
-  - meta
+  - pymdownx.highlight
   - pymdownx.superfences
   - pymdownx.tabbed:
       alternate_style: true