[Doc] add guide for how to monitor Pulsar stats

### 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?

Currently, we have descriptions for Pulsar stats, but we lack explanations on how to use them.

For example, sometimes the topic stats endpoint returns metrics that are suddenly 0, which is expected behavior.

### What is your suggestion?

Suggestions from @asafm:

- What are the admin APIs to use to obtain statistics
- What metrics are we emitting and what does each of them mean?
- what are the different configuration knobs for configuring the metrics
- Best practices

In addition, there are other suggestions made by @hangc0276 and @Anonymitaet:

To monitor Pulsar stats, we can use one of the following methods:

- pulsar-admin
- Pulsar manager
- Grafana (Prometheus)

So should we add the following content?

- Docs (Comparisons of these methods, e.g., When to use, Disadvantages, etc.)
- Tutorials (e.g., how to monitor the same Pulsar stats using these methods

### Any reference?

N/A

### Are you willing to submit a PR?

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

cc @momo-jun @DaveDuggins @D-2-Ed

Start by reviewing the existing Pulsar statistics documentation and the mentioned entry points: pulsar-admin, Pulsar Manager, and Grafana with Prometheus. Define the comparison and tutorial scope, then document how to obtain and interpret metrics, configure them, and explain expected zero values and monitoring best practices.

Written by the indexing model from the issue text.