[Doc] Add topics truncate doc

[gulecroc]

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• I searched in the issues and found nothing similar.

What issue do you find in Pulsar docs?

Using pulsar 4.0.2 and the pulsar-admin cli, I can use truncate function for topics.

I can't find it in the documentation https://pulsar.apache.org/docs/4.0.x/admin-api-topics/

What is your suggestion?

Add topics truncate documentation in https://pulsar.apache.org/docs/4.0.x/admin-api-topics/

Any reference?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!