Add Cloud Ledger Backend example on docs, Building Applications

[chowbao]

As part of docpocalypse a new 'Build Applications->Build custom network ingestion pipeline’ was created - stellar/stellar-docs#687

It shows how to use the ingestion sdk packages and the captive core ledger backend.

Create a sub-page under ‘Build Applications->Build custom network ingestion pipeline’ for 'Use Ledger Exporter instead of Captive Core’

Write a new docs page that explains how to modify the existing example to remove captive core ledger backend and use the BufferedStorageBackend configured for Google Cloud Storage instead. Highlight the benefits of this approach, such as lighter weight deployment, as captive core infrastructure no longer needed, etc.

Link to an existing docs source that provides context on the overall CDP design, something that shows the collaboration of LedgerExporter, DataStores like GCS, and LedgerBackends.