feat: add a switch to toggle light/dark theme

Add script for restoring preferred theme setting.
Update config for newer `mkdocs-material`.
Use native `mkdocs-material` styles where possible.

Closes: #45 #63

docs/js/index.js

@@ -0,0 +1,4 @@
+const settings = JSON.parse(localStorage.getItem(__prefix('__palette')));
+const scheme = settings?.color.scheme || (matchMedia('(prefers-color-scheme: dark)').matches ? 'slate' : 'default' );
+
+document.body.setAttribute('data-md-color-scheme', scheme);