feat(pricing): rates for the default DocsGPT model

$0.15 input, $0.50 output and $0.03 cached input per 1M tokens, so cost
budgets see usage of the default model instead of recording it at $0.

docsgpt/core/models/docsgpt.yaml

@@ -7,3 +7,6 @@ models:
     supports_tools: true
     attachments: [image]
     context_window: 1048576
+    input_cost_per_million: 0.15
+    output_cost_per_million: 0.5
+    cached_input_cost_per_million: 0.03