cost tracking (#1474)

rejigging old cost tracking docs + adding arbitrary cost tracking


https://langchain-5e9cc07a-preview-arbitr-1763594460-5fd94b2.mintlify.app/langsmith/cost-tracking

Author: tanushree-sharma

src/docs.json

@@ -970,6 +970,7 @@
                         "langsmith/sample-traces"
                       ]
                     },
+                    "langsmith/cost-tracking",
                     {
                       "group": "Advanced tracing techniques",
                       "pages": [
@@ -1005,8 +1006,7 @@
                     "langsmith/compare-traces",
                     "langsmith/share-trace",
                     "langsmith/platform-logs",
-                    "langsmith/data-export",
-                    "langsmith/calculate-token-based-costs"
+                    "langsmith/data-export"
                   ]
                 },
                 {