[ADD] describing fields in event.json file

[JuliusWelzel]

PR Description

As currently implemented, the events.json file being written does not contain information about the two required columns "onset" and "duration" in the .json file.

As those columns are REQURIED, I would propose adding some background information to the .json file every time events are written for people who are not familiar with the structure of the events.tsv files in.REQUIRED.

See #1254 for details.

• closes Add descriptions for onset and duration in event.json file #1254

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>"

[welcome]

Hello! 👋 Thanks for opening your first pull request here! ❤️ We will try to get back to you soon. 🚴🏽‍♂️

[sappelhoff]

Thanks @JuliusWelzel I merged main into your branch, so please pull before continuing to work on this (main contains some CI fixes now).

As for your PR, I am happy with the proposed changes, but please observe my comments in the code.

Finally, please follow the steps in the second to last section of this document: https://github.com/mne-tools/mne-bids/blob/main/CONTRIBUTING.md#instructions-for-first-time-contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.45%. Comparing base (87eea28) to head (3a37466).
Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1255      +/-   ##
==========================================
- Coverage   97.61%   97.45%   -0.17%     
==========================================
  Files          40       40              
  Lines        8685     8685              
==========================================
- Hits         8478     8464      -14     
- Misses        207      221      +14     

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[JuliusWelzel]

Hopefully all done, so should be good to go

[sappelhoff]

one last thing to do, the rest looks great -- thanks

[welcome]

🎉 Congrats on merging your first pull request! 🥳 Looking forward to seeing more from you in the future! 💪

[sappelhoff]

Thanks @JuliusWelzel!