[Nginx]: Add configuration examples for Nginx logs compatible with the integration in the documentation

### Integration Name

Nginx [packages/nginx]

### Dataset Name

nginx.access (and nginx.error)

### Integration Version

3.2.2

### Agent Version

9.4.4

### OS Version and Architecture

RHEL 9 x86_64

### User Goal

We are using the Nginx integration with Elastic Agent to collect and parse access and error logs. However, the current documentation lacks clear, practical examples of Nginx `log_format` configurations that are fully compatible with the integration's default parsing pipeline.

We would like to see official configuration examples added to the documentation or integration overview page that show:
- The recommended Nginx `log_format` directive to use with the integration.
- Examples for both access logs and error logs.

*Example of a custom log format that users might try:*
```
log_format custom '$remote_addr - $remote_user [$time_local] "$request" $status $body_bytes_sent "$http_referer" "$http_user_agent" $request_time $upstream_response_time';
```

Adding these examples would significantly improve the user experience and reduce prep time.

### Existing Features

The current documentation describes how to enable the integration and configure basic paths for log files, but it does not provide concrete `log_format` examples. Users are left to guess which fields are expected by the integration's ingest pipeline. It would be useful to have in hand a direct example.

### What did you see?

Currently, when users configure a custom `log_format` in Nginx that deviates from the default combined format, the integration may fail to parse logs correctly, resulting in:
- Missing or incorrectly mapped fields in Elasticsearch.
- Users having to reverse-engineer the integration's pipeline to understand the expected format.

### Anything else?

It would be very helpful if the documentation included:
1. The **default log format** that the integration expects (if any).
2. A **recommended `log_format`** that works out of the box with the integration with all features.
3. Examples of **common customizations** (e.g., adding response time, upstream status, etc.) and how to adjust the integration's processor configuration to handle them.

Start in the Nginx integration at packages/nginx and review its existing documentation, default parsing pipeline, and processor configuration. Add practical access and error log_format examples, including the expected default format and supported customization guidance. Done means users can configure Nginx logs without reverse-engineering the ingest pipeline.

Written by the indexing model from the issue text.