[Task]: Improve Enrichment docs

[damccorm]

What needs to happen?

There are a few targeted fixes needed for the Enrichment docs:

• https://beam.apache.org/documentation/transforms/python/elementwise/enrichment/ should mention BigQuery
• https://beam.apache.org/documentation/transforms/python/elementwise/enrichment/ should describe how we do batching and how we do caching (with_redis_cache)
• https://github.com/apache/beam/blob/master/examples/notebooks/beam-ml/bigtable_enrichment_transform.ipynb and https://github.com/apache/beam/blob/master/examples/notebooks/beam-ml/vertex_ai_feature_store_enrichment.ipynb should elaborate a little more on what a cross-join means in this context (maybe a picture would be nice?)
• In https://github.com/apache/beam/blob/master/examples/notebooks/beam-ml/bigtable_enrichment_transform.ipynb the example handler with composite row key support link is dead

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

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[damccorm]

@claudevdm this would be good to pick up at some point when you have space (don't drop other things, just when this fits in nicely)