Update mkdocs-material & add dark mode
In this MR:
- Update mkdocs-material to v7.x.x
- Fix custom extra stylesheets insertion - it fixes a bug with our theme on a non-existing page (404, e.g., https://docs.turris.cz/non-exist)
- Add color palette toggle (dark mode) - creates a color palette toggle between Turris & Slate themes