[FLINK-28765] Create a doc for protobuf format

[maosuhan]

After this feature has been done https://issues.apache.org/jira/browse/FLINK-18202, we should write a doc to introduce how to use the protobuf format in SQL.

[flinkbot]

CI report:

• b9b5e0d UNKNOWN
• de84447 Azure: SUCCESS
• 8e8c338 Azure: PENDING

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[MartijnVisser]

@maosuhan Thanks a lot for the docs! Some minor fixes, we do need to add Protobuf to the sql_connectors_yml file to make sure it's downloadable/embedded properly.

[MartijnVisser]

@maosuhan Thanks for the PR! I've added the needed data for displaying the Maven dependency and I've also copied the doc to the Chinese content directory. Anything to add from your end? Else I'll merge it.

[libenchao]

Thanks @maosuhan for contributing this, and @MartijnVisser for reviewing.
Do you think it's necessary to add some notes about the compiled pb classes. This is different from avro format because we need users to provide the compiled protobuf classes.

[MartijnVisser]

@libenchao I'm assuming we're talking about the .desc files a user should have?

[maosuhan]

@MartijnVisser @libenchao Thanks for your reviewing and I have updated the doc also I wrote more detailed information about how to use it. Anything I should change from your side?

[libenchao]

I'm assuming we're talking about the .desc files a user should have?

@MartijnVisser I mean the java classes compiled from the .desc file by protoc, we need users to do this step and provide the classes when submitting the flink job.

[maosuhan]

@libenchao thanks for the suggestion. Changes have been committed.

[libenchao]

LGTM, merging