Flink 20348 kafka avro connector examples

[sv3ndk]

What is the purpose of the change

• Provide additional examples of how to use the avro-confluent format. In particular, no example of how to serialize the kafka key or use the upsert connector are currently mentioned in the avro confluent connector documentation

Brief change log

• Added an example of a table serializing the kafka value in an avro record registered in the schema registry, while keeping the key in raw format
• Added an example of how to write to that table
• Added an example of a table using the schema registry for both the key and value
• Added an example of using the schema registry together with the kafka upsert connector

Only the English version of the documentation has been updated so far. Once the examples are approved, I'm happy to copy/paste them to the Chinese version of the documentation, at which point I'll need support for the few sentences that were updated/added.

Verifying this change

Manually executed ./build_docs.sh -p to check visually the result

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? no

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Automated Checks

Last check on commit afadbd4 (Fri May 28 08:06:58 UTC 2021)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!
• Invalid pull request title: No valid Jira ID provided

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Review Progress

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• ❓ 5. Overall code [quality] is good.

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[sv3ndk]

I'm realizing I used the wrong ticket number => closing and will re-open correctly