[Chore] Split the README between the engine and main game

The README is slowly expanding with new features and will likely become unwieldy soon. In order to alleviate this for future features, the README should be split into three parts:

1. **main** README (`./README.md`)
2. **engine** README (`./engine/README.md`)
3. **game** README (`./game/README.md`)

The **main** README will contain any details about the game and engine that are applicable to most people visiting for downloads or to understand the project. This should include links to resources, a game description, images/videos, build/download badges, and anything generally related to the overall project.

The **engine** README will contain any details that are _specific_ to the Entropy Engine itself. Most people would likely be focused on the overall project or the game itself. However, we should still have documentation specific to the game engine so that anyone that would like to use it, there will be instructions on how to do so. Additionally, the engine will likely be moved to a separate repository once it's in a stable state.

The **game** README will contain any details that are _specific_ to the Entropy game itself. Things in this README will be more in-depth to building/running the game. This will be more low-level than the **main** README for those that are more interested in contributing to the project.