add toolkitStackName to bootstrapping docs (#3749)

I spent a long time trying to figure out how to customise the name of the CDK bootstrap stack. Turns out it's quite easy.

www/docs/advanced/bootstrapping.md

@@ -89,6 +89,7 @@ There are two ways to customize the bootstrapping resources.
  qualifier: "my-team",
  fileAssetsBucketName: "my-team-CDKToolkit",
  customPermissionsBoundary: "my-team-pb",
+ toolkitStackName: "custom-cdk-bootstrap-stack-name",
  }
  }
  },
@@ -108,4 +109,4 @@ There are two ways to customize the bootstrapping resources.
  cdk bootstrap --template template.yaml
  ```
 
- When you run `cdk bootstrap`, SST will use the stack you've manually bootstrapped.
+ When you run `cdk bootstrap`, SST will use the stack you've manually bootstrapped.