Add Unity 3D documentation

[ninpl]

Feature request

We should add unity 3d documentation to devdocs. Online source: Unity Scripting Reference Trello Card: Ideas

Summary

Unity 3D is one of the 3 most used engines in the world. Note: This issue is more or less a reminder to a future myself to try to create the appropriate UrlScraper subclass to do the job. Meanwhile, if someone has the energy to do this, we can coordinate our efforts.

Examples