[Doc] Functions getting started no longer works

### 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?

I tried to follow https://pulsar.apache.org/docs/3.0.x/functions-quickstart/, but I ran into issues.

Specifically, when trying to run

```shell
bin/pulsar-admin functions create \
--function-config-file examples/example-function-config.yaml \
--jar examples/api-examples.jar
```

I got errors because the files were moved.

I see these files:
* `pulsar-functions/java-examples/target/pulsar-functions-api-examples.jar`
* `pulsar-functions/java-examples/src/main/resources/example-function-config.yaml`

### What is your suggestion?

I am not exactly sure what the right doc solution is. I tried to find something from the git history, but `git log --full-history -- examples/api-examples.jar` yields nothing.

If we do recommend using this jar `pulsar-functions/java-examples/target/pulsar-functions-api-examples.jar`, we'll need to mention that users must build the project to build the jar.

### Any reference?

_No response_

### Are you willing to submit a PR?

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

Start with the 3.0.x functions quickstart and compare its function creation command with the currently mentioned paths: pulsar-functions/java-examples/target/pulsar-functions-api-examples.jar and pulsar-functions/java-examples/src/main/resources/example-function-config.yaml. Verify whether the project must be built first, then update the documented command and prerequisites so the quickstart uses existing files and can be followed successfully.

Written by the indexing model from the issue text.