docs: reorganize self-host documentation structure (#776)

- Rename 'install' folder to 'self-host' for better clarity
- Move all self-hosting deployment files into the new self-host folder:
  - deploy-with-docker.mdx
  - deploy-with-kubernetes.mdx
  - deploy-with-aws-fargate.mdx
  - production-setup.mdx
  - upgrade.mdx
  - external-access.mdx
- Update docs.json navigation paths accordingly

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: d-bytebase

mintlify/docs.json

@@ -33,14 +33,14 @@
               {
                 "group": "Self-Host",
                 "pages": [
-                  "get-started/deploy-with-docker",
-                  "get-started/deploy-with-kubernetes",
-                  "get-started/deploy-with-aws-fargate",
-                  "get-started/install/external-postgres",
-                  "get-started/external-access",
-                  "get-started/install/external-url",
-                  "get-started/production-setup",
-                  "get-started/upgrade"
+                  "get-started/self-host/deploy-with-docker",
+                  "get-started/self-host/deploy-with-kubernetes",
+                  "get-started/self-host/deploy-with-aws-fargate",
+                  "get-started/self-host/external-postgres",
+                  "get-started/self-host/external-access",
+                  "get-started/self-host/external-url",
+                  "get-started/self-host/production-setup",
+                  "get-started/self-host/upgrade"
                 ]
               },
               "get-started/cloud",