docs: Auto-configure dark mode feature documented

[kratsg]

This resolves #23.

Add in documentation about how to configure this plugin for auto-configuring dark mode based on the CSS media feature prefers-color-scheme.

[fralau]

Thanks! Perhaps you want to expand a little on scheme:preference in material (explain what it does, and provide a link to exact point where this is documented)?

Since I am not totally familiar, I guess you refer to this: squidfunk/mkdocs-material#1266 (unless it's documented somewhere on Squidfunk's excellent doc for the material theme)?

[kratsg]

Thanks! Perhaps you want to expand a little on scheme:preference in material (explain what it does, and provide a link to exact point where this is documented)?

Good point, the documentation is here: https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/#color-scheme