[DOCS] Include POSIX compliant single quotes

[alenards]

Resolves #1930

Changes the pip install instruction to surround the -e argument with POSIX-compliant single quotes.

   git clone https://github.com/Kozea/WeasyPrint.git
   cd WeasyPrint
   python -m venv venv
   venv/bin/pip install -e '.[doc,test]'

[alenards]

@grewn0uille - I realize that I may need to reword my commit message. If there is anything I can adjust to align with the project's conventions, please let me know.

[grewn0uille]

Thanks! No worry, it’s fine :)