[Doc] Document how to write Window 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?

There is no documentation on how to write Window Functions

### What is your suggestion?

Add a page or a section in https://pulsar.apache.org/docs/next/functions-develop-api

### Any reference?

_No response_

### Are you willing to submit a PR?

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

Start by reading the functions-develop-api page linked in the issue and nearby Pulsar documentation to understand the existing structure and terminology. Done means the page or a new section explains how to write Window Functions and is included in the published documentation.

Written by the indexing model from the issue text.