feat: add 'coming soon' empty state for projects page & gitignore public dir

- Add animated coming-soon card when no projects exist
- Add Grafana shortcode and gebrauchtwagen-datenbank project
- Add hugo/eliaskohout.de/public/ to .gitignore and remove from tracking

hugo/eliaskohout.de/layouts/shortcodes/grafana.html

@@ -0,0 +1,13 @@
+<div class="grafana-embed">
+  <iframe
+    src="{{ .Get "url" }}"
+    width="{{ .Get "width" | default "100%" }}"
+    height="{{ .Get "height" | default "450" }}"
+    frameborder="0"
+    scrolling="no"
+    allowfullscreen
+    loading="lazy"
+    style="border: 1px solid rgba(255,255,255,0.1); border-radius: 6px; overflow: hidden;"
+    >
+  </iframe>
+</div>