Icon request: Motorbike

## Icon Request

* Icon name: motorbike
* Use case: Used when representing a motorcycle. Ideally it would be great to have one icon for each of the most famous bike types: sportbike, naked bike, motocross and adventure.
* Screenshots of similar icons:

No file, test, or entry point is named in the request. Start by reviewing how existing icons are defined and contributed in the repository, then determine the expected scope for a motorbike icon and whether the requested sportbike, naked bike, motocross, and adventure variants are separate additions; done means the agreed icon asset or assets represent the stated motorcycle use case.

Written by the indexing model from the issue text.