[MRG] migrate from .zenodo.json to CITATION.cff

[sappelhoff]

PR Description

closes #460

This will give us a nice "cite as" link next to the main repo landing page, pointing to the JOSS paper. Furthermore we retain all features that .zenodo.json gave us* ... so this is an upgrade.

*except the links to the mne-bids posters ... there is no concept of related_identifiers in CITATION.cff ... or maybe I misused it previously in Zenodo.

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to .zenodo.json
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #1037 (4bf2703) into main (491ac8d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1037   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files          25       25           
  Lines        3787     3787           
=======================================
  Hits         3605     3605           
  Misses        182      182           

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