docs: Add missing SETTING_UP_SKILLS status to Cloud API documentation

Added the SETTING_UP_SKILLS status which was discovered during API testing.
This status appears when the agent is configuring skills and tools after
the sandbox is ready but before the conversation is fully ready.

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

openhands/usage/cloud/cloud-api.mdx

@@ -127,6 +127,7 @@ The `status` field indicates the current state of the conversation startup proce
 - `WORKING` - Initial processing
 - `WAITING_FOR_SANDBOX` - Waiting for sandbox to be ready
 - `PREPARING_REPOSITORY` - Cloning and setting up the repository
+- `SETTING_UP_SKILLS` - Configuring agent skills and tools
 - `READY` - Conversation is ready to use
 - `ERROR` - An error occurred during startup