Freeze docs version for 21.09.0

docs/site/freeze_version.sh

.../site/versioned_docs/version-21.09.0/compare-to-others.md

docs/site/versioned_docs/version-21.09.0/consumer-clients.md

...sioned_docs/version-21.09.0/cookbook/acknowledgements.mdx

...site/versioned_docs/version-21.09.0/cookbook/datasets.mdx

...site/versioned_docs/version-21.09.0/cookbook/metadata.mdx

...e/versioned_docs/version-21.09.0/cookbook/next_stream.mdx

.../site/versioned_docs/version-21.09.0/cookbook/overview.md

docs/site/versioned_docs/version-21.09.0/cookbook/query.mdx

...rsioned_docs/version-21.09.0/cookbook/simple-consumer.mdx

...rsioned_docs/version-21.09.0/cookbook/simple-pipeline.mdx

...rsioned_docs/version-21.09.0/cookbook/simple-producer.mdx

.../site/versioned_docs/version-21.09.0/core-architecture.md

docs/site/versioned_docs/version-21.09.0/data-in-asapo.md

docs/site/versioned_docs/version-21.09.0/getting-started.mdx

docs/site/versioned_docs/version-21.09.0/overview.md

docs/site/versioned_docs/version-21.09.0/p02.1.md

docs/site/versioned_docs/version-21.09.0/producer-clients.md

docs/site/versioned_examples/version-21.09.0/c/consume.c

docs/site/versioned_examples/version-21.09.0/cleanup.sh