Extend 'iox2-config' to describe the config parameter

## Refactoring

The `config/README.md` has a detailed description of the config parameter. It would be nice to get the same info with `iox2 config explain`.

## Tasks

* [ ] create a `iox2 config explain` command which output the same content as `config/README.md`
* [ ] add a CI check to enforce that the output of `iox2 config explain` is in sync with `config/README.md`