Add noop alloydb doc for publishing

[evantahler]

#34624 didn't work because there was a missing doc. Publishing now requires docs.

• https://storage.cloud.google.com/airbyte-ci-reports-multi/airbyte-ci/connectors/publish/master/master/1706555680/3bec4f0dec1e15feac5dca4adaf0d6264573ec64/source-alloydb/3.1.8/output.html

Running validator: validate_all_tags_are_keyvalue_pairs
Running validator: validate_at_least_one_language_tag
Running validator: validate_major_version_bump_has_breaking_change_entry
Running validator: validate_docs_path_exists
Running validator: validate_metadata_base_images_in_dockerhub
Running validator: validate_pypi_only_for_python
Running validator: validate_metadata_images_in_dockerhub
Checking that the following images are on dockerhub: [('airbyte/source-alloydb', '3.1.8')]
Local airbyte-integrations/connectors/source-alloydb/metadata.yaml md5_hash: TBljBXffLMW7NTaVjXZqog==
Remote metadata/airbyte/source-alloydb/3.1.8/metadata.yaml md5_hash: TBljBXffLMW7NTaVjXZqog==
The metadata file could not be uploaded: Expected to find connector doc file at docs/integrations/sources/alloydb.md, but none was found.[0m

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Name	Status	Preview	Updated (UTC)
airbyte-docs	🔄 Building (Inspect)	Visit Preview	Jan 29, 2024 11:40pm

[evantahler]

/approve-and-merge reason="adding noop doc for removed destination"

[octavia-approvington]

All in!!