Document `page-numbering` for Typst

[cwickham]

Since 5c56f77 you can do:

---
title: "Untitled"
author: Charlotte Wickham
format: 
  typst:
    page-numbering: ""
---

## Introduction

{{< lipsum 2 >}}

## Section

{{< lipsum 3 >}}

But we don't document it, either in the Reference or Guide > Documents > Typst > Typst Basics