Add minimal landing page for root endpoint `/`

### What would you like to be added?

Currently accessing the root endpoint `/` returns:

{
"error": "FF10109: Not found"
}

However FireFly exposes useful interfaces:

/ui
/api
/sandbox

It might improve developer experience if the root endpoint
returned a simple landing page or JSON response linking
to these interfaces.

Example:

FireFly Node Running

UI → /ui
API Docs → /api
Sandbox → /sandbox

### Why is this needed?

This can user experience and overall project enhancment