fix(docs): remove dead references to railway, render and northflank

Remove references in the navigation to deployment pages that do not exist:
- railway.md
- render.md
- northflank.md

These pages were listed in docs.json but the files do not exist
in any of the languages (en, es, pt-BR, zh-CN), causing broken links
in the documentation.

Fixes issues identified in the review of PR #14415.

docs/docs.json

@@ -1618,10 +1618,7 @@
                   "es/platforms/hetzner",
                   "es/platforms/gcp",
                   "es/platforms/macos-vm",
-                  "es/platforms/exe-dev",
-                  "es/railway",
-                  "es/render",
-                  "es/northflank"
+                  "es/platforms/exe-dev"
                 ]
               },
               {
@@ -2111,10 +2108,7 @@
                   "pt-BR/platforms/hetzner",
                   "pt-BR/platforms/gcp",
                   "pt-BR/platforms/macos-vm",
-                  "pt-BR/platforms/exe-dev",
-                  "pt-BR/railway",
-                  "pt-BR/render",
-                  "pt-BR/northflank"
+                  "pt-BR/platforms/exe-dev"
                 ]
               },
               {