[Bug] OKF bundles index/subdirectories pointing to nowhere/null

Taking samples/ga4 as an example, after I pushed the bundle to GCP Knowledge Catalog. Then navigate to ga4 root index I could see 3 Subdirectories:

- [datasets](https://pantheon.corp.google.com/datasets/index.md) - A sample of obfuscated Google Analytics BigQuery event export data for three months from the Google Merchandise Store is available as a public dataset in BigQuery.
- [references](https://pantheon.corp.google.com/references/index.md) - This directory contains specifications for data joins and definitions for user activity and purchase metrics.
- [tables](https://pantheon.corp.google.com/tables/index.md) - Contains Google Analytics event export data from the ga4_obfuscated_sample_ecommerce dataset.

Click any of them will redirect to "URL not found" page.

Mouse over tables shows the link: https://console.cloud.google.com/tables/index.md

Reproduce the published samples/ga4 bundle and inspect the generated ga4 root index and its datasets, references, and tables links. Confirm that the links no longer point to pantheon.corp.google.com or console.cloud.google.com paths that return “URL not found” in Google Cloud Knowledge Catalog.

Written by the indexing model from the issue text.