docs: Add Obsidian client via the Agent Client plugin (#220)

Author: RAIT-09

docs/overview/clients.mdx

@@ -15,3 +15,5 @@ The following clients can be used with an ACP Agent:
   - through the [yetone/avante.nvim](https://github.com/yetone/avante.nvim) plugin
 - [Sidequery _(coming soon)_](https://sidequery.dev)
 - [Web Browser with AI SDK](https://github.com/mcpc-tech/ai-elements-remix-template) (powered by [@mcpc/acp-ai-provider](https://github.com/mcpc-tech/mcpc/tree/main/packages/acp-ai-provider))
+- [Obsidian](https://obsidian.md)
+  - through the [Agent Client](https://github.com/RAIT-09/obsidian-agent-client) plugin