Add main website headings on mobile sidebar

### Describe the bug

On https://toga.beeware.org/en/latest/, there is a navigation drawer button when screen size is small. It can be opened:

However, the navigation bar items from the main page are not included:

### How to reproduce the bug

Go to https://toga.beeware.org/en/latest/ with a small screen

### Minimum example code

```shell

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### Screenshots

_No response_

### Environment details

- Operating system and version:
- Python version:
- Software versions:
- Briefcase:
- Toga:
- ...

### Logs

```

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### Additional context

_No response_

Start at https://toga.beeware.org/en/latest/ and inspect the documentation site's navigation behavior at a small screen size. Compare the open mobile drawer with the main-page navigation shown in the issue; done means the main website headings are also available in the mobile sidebar.

Written by the indexing model from the issue text.