Migrate Practicalli Neovim to MkDocs Material version 9

[practicalli-johnny]

Update Material for MkDocs to version 9

The GitHub Publish Book workflow installs version 9 of Material for MkDocs

• update mkdocs material (done in Migrate Practicalli Clojure to MkDocs Material version 9 clojure#441)
• update mkdocs features

Install Material for MkDocs version 9 locally

pip install mkdocs-material=="9.*" 

Update mkdocs.yml configuration to include features that became optional in version 9

    - content.code.copy
    - content.action.edit
    - content.action.view
    - navigation.footer

Reference

• https://squidfunk.github.io/mkdocs-material/upgrade/