Restructuring the Unity SDK

[endel]

Mitch (@mitchLucid) and Erik (@LS-Erik) have been working on an update to bring more Unity-friendly tooling to the Unity client.

New "Getting started" page

• Preview: https://deploy-preview-58--colyseus-docs.netlify.app/getting-started/unity3d-client/
• Documentation PR: https://github.com/colyseus/docs/pull/58

Feedback is very welcome!