[Doc] [Admin REST API] Missing examples for registering sinks, sources, functions

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### What issue do you find in Pulsar docs?

In the Admin REST API docs, there is no real good example of how to register a sink by encoding data as `multipart/form-data`.

This affects the following methods:
https://pulsar.apache.org/sink-rest-api/?version=master#operation/registerSink
https://pulsar.apache.org/functions-rest-api/?version=master#operation/registerFunction
https://pulsar.apache.org/source-rest-api/?version=master#operation/registerSource

If you click on the request samples `text/plain` tab we see the following description:
```
Examples
1. Create a JSON object
{
"inputs": "persistent://public/default/input-topic",
"parallelism": "4",
"output": "persistent://public/default/output-topic",
"log-topic": "persistent://public/default/log-topic",
"classname": "org.example.test.ExclamationFunction",
"jar": "java-function-1.0-SNAPSHOT.jar"
}
2. Encapsulate the JSON object to a multipart object (in Python)
from requests_toolbelt.multipart.encoder import MultipartEncoders
mp_encoder = MultipartEncoder([('functionConfig',(None, json.dumps(config), 'application/json'))])
```

### What is your suggestion?

We want to suggest replacing the existing description that uses python with a simple example using curl (a tool that is already on most systems by default). An example for `registerSink` would look like this:

```
curl -X "POST" "https:///admin/v3/sinks///" \
-H "Accept: application/json" \
-H "Authorization: Bearer " \
-H 'Content-Type: multipart/form-data; charset=utf-8; boundary=__X_PAW_BOUNDARY__' \
-F "sinkConfig={\"className\":\"org.example.MySinkTest",\"inputs\":[\"persistent://public/default/sink-input\"],\"configs\":{\"sinkConfigKey\":\"myExampleValue\"}};type=application/json"
```

### Any reference?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start with the registerSink, registerFunction, and registerSource operations linked in the issue, then inspect their text/plain request samples. Replace the Python multipart explanation with curl examples for each operation, and verify that the rendered Admin REST API documentation shows complete multipart/form-data requests.

Written by the indexing model from the issue text.