Add some documentation that explains architecture of project for contributers

[mathijshoogland]

Describe the feature: I am trying to contribute to Kibana and spent quite some time in understanding how the project is exactly setup. I think this should be added to the documentation since this makes it easier for developers to contribute to the project. I made the following image for my understanding, maybe we could add this or an updated image in the contributing.md file?

[BjornvdLaan]

I think this is a great idea!

I've been looking at Kibana for a while, and find it sometimes difficult to wrap my head around it. Also, there exists a lot of literature that suggest that absence of such documentation can be an entry barrier for newcomers to open source projects. Therefore, it could be of great value.

[KOTungseth]

We have an internal initiative for implementing our dev docs. For more information, refer to #77958.