Package citation should be replaced by a DOI

Currently the package citation reference key is generate after the publication of the stable version of the documentation. This lead to citations on the version N-1 instead of citation on the version N.

This issue could be fixed with a reserved DOI that will be updated after SWH release id.

This DOI need to be propagated to README.md, docs/index.md, CITATION.cff, metacode.json and planetary-coverage.bib files.

A reference in the CHANGELOG.md should be added as well.

This change might be performed by bumpversion eventually.