docs: Merge browser pages into single Kernel integration page

Combines browser-use.mdx and agent-browser.mdx into one page at
docs/integrations/kernel.mdx. Adds new Integrations section to sidebar.

Author: beran-t

docs.json

@@ -47,15 +47,13 @@
             "pages": [
               "docs/use-cases/coding-agents",
               "docs/use-cases/computer-use",
-              "docs/use-cases/ci-cd",
-              {
-                "group": "Browser use",
-                "icon": "globe",
-                "pages": [
-                  "docs/use-cases/browser-use",
-                  "docs/use-cases/agent-browser"
-                ]
-              }
+              "docs/use-cases/ci-cd"
+            ]
+          },
+          {
+            "group": "Integrations",
+            "pages": [
+              "docs/integrations/kernel"
             ]
           },
           {