Make configuration page point to all relevant configuration pages

[utnapischtim]

[fenekku]

In preparation for v13, I am cleaning up those v12 issues.

I think changing the configuration page https://github.com/inveniosoftware/docs-invenio-rdm/blob/master/docs/reference/configuration.md to list the configuration page of each relevant repository instead is the most foolproof and effortless approach here (with least repetition).