Added support for prefers-color-scheme in docs

Now, instead of always defaulting to light theme, documentation, docs
will default to the color scheme based by the user preference (reported
by web browser as a `prefers-color-scheme` media query).

It is still possible for user to change the theme by using the switch
next to the search box. His preference will be remembered.

docs/docusaurus.config.js

@@ -10,6 +10,9 @@ module.exports = {
   projectName: "zmk", // Usually your repo name.
   plugins: [path.resolve(__dirname, "src/docusaurus-tree-sitter-plugin")],
   themeConfig: {
+    colorMode: {
+      respectPrefersColorScheme: true,
+    },
     googleAnalytics: {
       trackingID: "UA-145201102-2",
       anonymizeIP: true,