docs(admin-api-topics): document topic truncate operation (#1121)

Add a "Truncate topic" section in admin-api-topics.md with the
standard pulsar-admin / REST API / Java tab layout, matching the
style of the surrounding Unload/Delete sections.

The feature itself has been available for years via
`pulsar-admin topics truncate` (CmdTopics.TruncateCmd), the
Topics#truncate(String) Java admin API, and
`DELETE /{tenant}/{namespace}/{topic}/truncate` on the broker
(PersistentTopics#truncateTopic). It just wasn't documented on
pulsar.apache.org/docs/*/admin-api-topics/.

Fixes apache/pulsar#24086

Author: singhvishalkr

docs/admin-api-topics.md

@@ -257,6 +257,38 @@ admin.topics().unload(topic);
 </Tabs>
 ````
 
+### Truncate topic
+
+You can truncate a topic in the following ways. The truncate operation moves all cursors to the end of the topic and deletes all inactive ledgers, freeing the storage used by messages that have already been consumed.
+
+````mdx-code-block
+<Tabs groupId="api-choice"
+  defaultValue="pulsar-admin"
+  values={[{"label":"pulsar-admin","value":"pulsar-admin"},{"label":"REST API","value":"REST API"},{"label":"Java","value":"Java"}]}>
+<TabItem value="pulsar-admin">
+
+```shell
+pulsar-admin topics truncate persistent://test-tenant/ns1/tp1
+```
+
+</TabItem>
+<TabItem value="REST API">
+
+[](swagger:/admin/v2/PersistentTopics_truncateTopic)
+
+</TabItem>
+<TabItem value="Java">
+
+```java
+String topic = "persistent://my-tenant/my-namespace/my-topic";
+admin.topics().truncate(topic);
+```
+
+</TabItem>
+
+</Tabs>
+````
+
 ### Get stats
 
 For the detailed statistics of a topic, see [Pulsar statistics](administration-stats.md#topic-stats).