chore: remove blog, open-source and news sections

- Delete the blogs, opensource-projects and notifications trees
- Drop their nav entries and i18n keys
- Point the nav at the new open-source site instead
- Drop the opensource-projects sitemap priority

docs/en-gb/_nav.json

@@ -14,19 +14,8 @@
     "link": "/services/",
     "activeMatch": "/services/"
   },
-  {
-    "text": "nav.insights",
-    "link": "/blogs/",
-    "activeMatch": "/blogs/"
-  },
-  {
-    "text": "nav.news",
-    "link": "/notifications/",
-    "activeMatch": "/notifications/"
-  },
   {
     "text": "nav.opensource",
-    "link": "/opensource-projects/",
-    "activeMatch": "/opensource-projects/"
+    "link": "https://opensource.onixbyte.com"
   }
 ]