feat(favicon): add favicon and browser icons for Issue #36

- favicon.ico (16x16 + 32x32)
- favicon-32x32.png, favicon-16x16.png
- apple-touch-icon.png (180x180)
- site.webmanifest
- Linked in index.php head

Resolves #36

bilder/favicon/site.webmanifest

@@ -0,0 +1,10 @@
+{
+  "name": "Haus Schleusingen",
+  "short_name": "HS",
+  "icons": [
+    { "src": "/bilder/favicon/favicon-32x32.png", "sizes": "32x32", "type": "image/png" },
+    { "src": "/bilder/favicon/favicon-16x16.png", "sizes": "16x16", "type": "image/png" }
+  ],
+  "theme_color": "#1c1917",
+  "background_color": "#fafaf9"
+}