[FEATURE] Document compositing multiple applications and multi-file design

**Is your feature request related to a problem? Please describe.**

In the routing docs we need to explain how to bring in routes from different modules or applications into the main route.

**Describe the solution you'd like**

Create a clear, concise, interesting example. Reference FastHTML Gallery and other projects that leverage this technique.

**Implementations**

1. [FastHTML Gallery](https://github.com/AnswerDotAI/fasthtml-gallery)

**Problem solved**
Explain what problem this feature would solve and how it would benefit users of the library:

**Additional context**
Add any other context or screenshots about the feature request here.

**Confirmation**
Please confirm the following:
- [x] I have checked the existing issues and pull requests to ensure this feature hasn't been requested before.
- [x] I have read the project's documentation to ensure this feature doesn't already exist.

Start with the existing routing documentation and review the FastHTML Gallery example linked in the issue, along with other referenced projects if available. The work is complete when the docs clearly explain combining routes from multiple modules or applications into a main route, using a concise example and including the relevant references.

Written by the indexing model from the issue text.