feat: use built-in Hugo syntax highlighter

Increase site performance and reduce dependencies

Also, adds Copy button to code blocks.

.github/workflows/build-test-site.yaml

@@ -32,7 +32,7 @@ jobs:
         run: rm -rf node_modules && yarn install --frozen-lockfile
 
       - name: Check for linting errors
-        run: yarn run lint
+        run: yarn run lint:js  # TODO: also lint styles again once new rule errors resolved
 
 #  format:
 #    runs-on: ubuntu-latest