Document core API group

[afritzler]

Summary

Document the types usage in the concepts/core section of the project documentation. The documentation should give a use enough information on how to create a certain type and describe the various fields. A simple example should be provided. At the end we should reference the e2e example we have in the config folder which illustrate more complex scenarios.

[afritzler]

/ref https://ironcore-dev.github.io/metal-operator/