Production-ready default Kafka configurations

[refset]

E.g. default :replication-factor should probably be 3 for typical production usage here https://github.com/juxt/crux/blob/69b3078c5c2d0c28c45c6a53f7da7fbb34c26217/crux-core/src/crux/bootstrap.clj#L34 (or wherever this ends up after the config branch is merged #340)

Additionally Confluent Cloud recommends 6 partitions by default, which seems like a sensible default for the document topic. See: https://github.com/juxt/crux/blob/ee310eb5cf153a8b4ad3d8c5764da7aa0bdc0669/docs/example/crux-cloud/README.adoc

There may also be other useful default config options of interest. Somewhat related is the ability to POST to remote Crux nodes for Kafka configuration rather having to think about making .properties files available.

[hraberg]

When making this change, sanity check that the tests and dev.clj, or rather, anything related to embedded kafka uses a replication factor of 1.

[jarohen]

we have these in Soak, let's ensure these are documented as part of #386