📝 docs(citations): promote nf-core to top-level citation section

Move the nf-core framework citation out of the plugin entry and into
its own section, consistent with the Nextflow citation above it.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: FloWuenne

CITATIONS.md

@@ -4,12 +4,14 @@
 
 > Di Tommaso P, Chatzou M, Floden EW, Barja PP, Palumbo E, Notredame C. Nextflow enables reproducible computational workflows. Nat Biotechnol. 2017 Apr 11;35(4):316-319. doi: 10.1038/nbt.3820. PubMed PMID: 28398311.
 
+## [nf-core](https://www.nature.com/articles/s41587-020-0439-x)
+
+> Ewels PA, Peltzer A, Fillinger S, Patel H, Alneberg J, Wilm A, Garcia MU, Di Tommaso P, Nahnsen S. The nf-core framework for community-curated bioinformatics pipelines. Nat Biotechnol. 2020 Mar;38(3):276-278. doi: 10.1038/s41587-020-0439-x.
+
 ## Nextflow plugins used by this pipeline
 
 - [`nf-core-utils@0.4.0`](https://github.com/nf-core/nf-utils)
 
-  > Ewels PA, Peltzer A, Fillinger S, Patel H, Alneberg J, Wilm A, Garcia MU, Di Tommaso P, Nahnsen S. The nf-core framework for community-curated bioinformatics pipelines. Nat Biotechnol. 2020 Mar;38(3):276-278. doi: 10.1038/s41587-020-0439-x.
-
   Software source pinned in `nextflow.config`: `https://github.com/nf-core/nf-utils` (version `0.4.0`).
 
 - [`nf-schema@2.3.0`](https://github.com/nextflow-io/nf-schema)