docs(custom-resources): fix dead links in custom-resources package documentation (#29599)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

The links to the example files at https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.custom_resources-readme.html#provider-framework-examples are no longer valid since the repository underwent restructuring.

### Description of changes

Replaces the URLs to the S3File and S3Assert example files with valid GitHub links

### Description of how you validated changes

Visited the updated links

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Booligoosh

packages/aws-cdk-lib/custom-resources/README.md

@@ -317,8 +317,8 @@ This module includes a few examples for custom resource implementations:
 
 Provisions an object in an S3 bucket with textual contents. See the source code
 for the
-[construct](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/custom-resources/test/provider-framework/integration-test-fixtures/s3-file.ts) and
-[handler](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/custom-resources/test/provider-framework/integration-test-fixtures/s3-file-handler/index.ts).
+[construct](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk-testing/framework-integ/test/custom-resources/test/provider-framework/integration-test-fixtures/s3-file.ts) and
+[handler](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk-testing/framework-integ/test/custom-resources/test/provider-framework/integration-test-fixtures/s3-file-handler/index.ts).
 
 The following example will create the file `folder/file1.txt` inside `myBucket`
 with the contents `hello!`.
@@ -348,8 +348,8 @@ This sample demonstrates the following concepts:
 
 Checks that the textual contents of an S3 object matches a certain value. The check will be retried
 for 5 minutes as long as the object is not found or the value is different. See the source code for the
-[construct](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/custom-resources/test/provider-framework/integration-test-fixtures/s3-assert.ts)
-and [handler](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/custom-resources/test/provider-framework/integration-test-fixtures/s3-assert-handler/index.py).
+[construct](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk-testing/framework-integ/test/custom-resources/test/provider-framework/integration-test-fixtures/s3-assert.ts)
+and [handler](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk-testing/framework-integ/test/custom-resources/test/provider-framework/integration-test-fixtures/s3-assert-handler/index.py).
 
 The following example defines an `S3Assert` resource which waits until
 `myfile.txt` in `myBucket` exists and includes the contents `foo bar`: