Add documentation how a feature works

[MalteJ]

We should probably move the example feature from gardenlinux (https://github.com/gardenlinux/gardenlinux/tree/main/features/example) to this repo.
The README.md of features/example still holds outdated references about Makefiles. This should be removed.
We should provide a tutorial/documentation how to create a new feature.
It's also important to understand how the builder works with the different files within a feature. In which order is everything executed? What does the builder in addition to the scripts in a feature directory?

We need to explain a little bit better the builder's conventions.

[MalteJ]

Maybe the example feature should be included in the builder repo itself?

[MalteJ]

Why are there exec.pre, exec.config and exec.post when all three are executed one after the other? I could write everything into one file!?

[nkraetzschmar]

Why are there exec.pre, exec.config and exec.post when all three are executed one after the other? I could write everything into one file!?

The reason for multiple exec steps is to provide ordering of steps across multiple features. i.e. exec.late will only run after all features exec.conf ran, so it can be used for clean up of logs etc.