Explain CI and Unity related topics like how to set up tests

**Is your feature request related to a problem? Please describe.**

It is a steep learning curve to get started with Unity and CI concepts by themselves, let alone the little details about the combination of both.

On top of that there seems to be no good (free) source of documentation except on youtube that can help find proper real world examples on how to integrate "continuous integration pipelines" with their unity project.

**Describe the solution you'd like**

In order to help as many people as possible to succeed in creating their first or nth CI pipeline, it will probably be very useful to create some documentation that is well written and stays to the point (no more no less) about how to set up certain concepts.

Concepts could be:
- Project settings to keep in mind when building in CI
- How to set up assembly files for your project (including the test ones)
- How to test your project
- A good example of an editor test
- A good example of a playmode test

(these example tests should be easy to understand and relatable as real world example)

My suggestion would be to create a section called Unity, and put any subjects in there.

**Describe alternatives you've considered**
- The first place I looked was the Unity forums, but I was unable to find high quality answers. Also many links seem to be broken.
- I have tried to find good resources online, but found that the proper tutorials and examples are from unity themselves and behind a paywall. Even then they are video tutorials, where you can't quickly grab the yeast of it and get started with implementation.
- I tried looking for other open sources, but found that open source in game development (and especially for unity) is still in it's very early stages at this point.