docs: remove Custom API Endpoint secret manager integration (#920)

Remove documentation for the Custom API Endpoint feature from the secret
manager integration section. This feature is being deprecated.

Changes:
- Removed "Custom API Endpoint" from supported integrations list
- Removed complete Custom API Endpoint documentation section including
  endpoint format and expected response examples

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

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

Author: d-bytebase

mintlify/get-started/connect/overview.mdx

@@ -99,7 +99,6 @@ Never store database passwords in Bytebase. Use your existing secret manager for
 - **[AWS Secrets Manager](/get-started/connect/aws#aws-secrets-manager)** - For AWS deployments
 - **[GCP Secret Manager](/get-started/connect/gcp#gcp-secret-manager)** - For GCP deployments
 - **HashiCorp Vault** - Enterprise secret management
-- **Custom API Endpoint** - Custom integrations
 
 #### HashiCorp Vault
 
@@ -118,19 +117,6 @@ Requires Vault KV v2 engine
 2. Choose authentication: Token or AppRole
 3. Specify secret location
 
-#### Custom API Endpoint
-
-**Endpoint Format:** `{{http://example.com/secrets/mydbkey}}`
-
-**Expected Response:**
-```json
-{
-  "payload": {
-    "data": "base64_encoded_password"
-  }
-}
-```
-
 ## Instance Management
 
 Configure instance-level settings to optimize how Bytebase interacts with your databases.