Add main website headings on mobile sidebar

[johnzhou721]

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

Screenshots

No response

Environment details

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

Logs

Additional context

No response