Update branding to new Semantica logo

- Rename logo PNG to semantica-logo.png (lowercase, hyphenated)
- Update docs.json logo (light/dark) and favicon to reference new PNG
- Replace legacy purple favicon with new teal brain neural network icon

docs/docs.json

@@ -10,8 +10,8 @@
   },
   "favicon": "/assets/img/semantica-logo.png",
   "logo": {
-    "light": "/assets/img/semantica-wordmark-light.svg",
-    "dark": "/assets/img/semantica-wordmark-dark.svg"
+    "light": "/assets/img/semantica-logo.png",
+    "dark": "/assets/img/semantica-logo.png"
   },
   "font": {
     "headings": {