[Doc] add golang transaction demo in txn-use.md

[geniusjoe]

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• I searched in the issues and found nothing similar.

What issue do you find in Pulsar docs?

There are some developers using golang sdk txn feature, and there is few txn related demos in pulsar official docs.
apache/pulsar-client-go#1222

Since there is only a java sdk demo in transaction doc, we could add a golang sdk demo as well.

What is your suggestion?

I wrote a golang demo pr: apache/pulsar-site#948 .

Please take a look and feel free to give me advice.

Any reference?

This demo primarily references golang txn test cases and java txn demo.

Are you willing to submit a PR?

• I'm willing to submit a PR!