docs: remove outdated manual sync instance schema section (#844)

The "Sync instance schema" section is no longer accurate as Bytebase now automatically synchronizes the instance schema upon instance creation. Manual sync is no longer required for newly added instances.

This removes unnecessary steps from the onboarding documentation and prevents user confusion about when schema synchronization occurs.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: d-bytebase

mintlify/get-started/step-by-step/add-an-instance.mdx

@@ -23,11 +23,3 @@ Go to **Instances** and **Add Instance**.
 ![bb-instances-2](/content/docs/get-started/step-by-step/add-an-instance/bb-instances-2.webp)
 
 For **Host or socket**, the screenshot assumes you run Bytebase inside Docker and try to connect to a database instance on the same host, thus it uses `host.docker.internal`. Check out [Instance Connection Configuration](/get-started/instance) for the detailed guide.
-
-## Sync instance schema
-
-After instance is added, click **Sync Now** to synchronize the instance schema such as its containing databases to Bytebase.
-
-![bb-instance-sync](/content/docs/get-started/step-by-step/add-an-instance/bb-instance-sync.webp)
-
-By default, Bytebase doesn't automatically sync the instance schema. You can set a [Scan Interval](/administration/instance/#scan-interval) to ask Bytebase to sync the schema periodically.