ECS Documentation

First of all, I might have put the cart before the horse with this one - sorry for that!

I am interested in the ECS powering Stride, but I was unable to find some good documentation about how it works.
Most helpful was actually a forum post from 2018, but that also only discussed part of it.

So after hopping on discord and having a nice chat with some folks, I ended up with the impression "documentation about it is not great at the moment, so anything I find until I get it to work, I can just write down and it will be an improvement".
So I did that, wrote two markdown documents about it.

Then I read the contribution guidelines and the whole process.

So, again, sorry for not following the process there - but hey! Maybe you still want it, because we don't have docs about it yet. I'm gonna create a PR and link it here immediately after finishing writing this.

Notes
A) What added to my impression is a currently existing page about the subject has some outdated code - EntityManager.Entities does not exist in 4.1.0.1898.

B) I added two pages. While one is just a technical description of what there is , the other one is a write-up about ECS in general, explaining the concept aimed at an audience that doesn't know i yet.

C) I also added a quick diagram to make the decision easier for inexperienced users. I just used diagrams.net, didn't know if there's another standard for these or something.

Start with the existing managing-entities page and the contribution guidelines, then review the two proposed Markdown pages and their ECS diagram. Check the documentation build instructions locally and verify that the ECS guidance matches Stride 4.1.0.1898, including the outdated EntityManager.Entities example, before the documentation is accepted.