Misc maintenance

[sappelhoff]

PR Description

Most importantly, I added a short section t o the contributing guide, explicitly stating what we followed anyhow: That we support the two most recent stable releases of mne. More versions MAY work, but we won't fix behavior there if they don't.

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 CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.46%. Comparing base (87eea28) to head (b827f02).
Report is 61 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1299      +/-   ##
==========================================
- Coverage   97.61%   97.46%   -0.16%     
==========================================
  Files          40       40              
  Lines        8685     8780      +95     
==========================================
+ Hits         8478     8557      +79     
- Misses        207      223      +16     

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