update changelog

Author: github-actions[bot]

docs/changelog/changelog.mdx

@@ -4,6 +4,12 @@ icon: "clock"
 iconType: "solid"
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+<Update label="v0.40.0" description="February 26, 2025">
+### [Adds Chat Agent and subdirectory support in codegen.](https://github.com/codegen-sh/codegen-sdk/releases/tag/v0.40.0)
+- Add functionality to specify subdirectories in `codegen.function`
+- Introduce a new Chat Agent
+</Update>
+
 <Update label="v0.39.1" description="February 26, 2025">
 ### [Custom codebase path setting for code generation.](https://github.com/codegen-sh/codegen-sdk/releases/tag/v0.39.1)
 - Allow setting custom codebase path in code generation