DOC: Improve first time contributor welcome message

[jhlegarreta]

Improve first time contributor welcome message in GitHub Actions workflow file:

• Improve the readability by separating paragraphs with empty lines, using a numbered list, using syntax highlighting (bold faces, links).
• Reword some passages to provide a more compact message, potentially improving the effectiveness of the message.
• Remove the mention to Russ as an author as he is not listed in the .zenodo.json file.
• Add emojis to make it more visually appealing.

Modify the CONTRIBUTING.md file to point to the Contributing guidelines section of the NiPreps repository.

Changes proposed in this pull request

(see the above commit message).

Documentation that should be reviewed

Does not apply to the PR.

[welcome]

Welcome, new contributor!
We ask you to read through the Contributing Guide: https://github.com/nipreps/fmripost-template/blob/main/CONTRIBUTING.md
These are guidelines intended to make communication easier by describing a consistent process, but don't worry if you don't get it everything exactly "right" on the first try.
To boil it down, here are some highlights:

1. Consider starting a conversation in the issues list before submitting a pull request. The discussion might save you a lot of time coding. 2) Please use a descriptive title for your pull request. Please remember to set your pull request as a "draft" if it is not ready for review. 3) Any code you submit will be licensed under the same terms (Apache License 2.0) as the rest of fMRIPost-template. 4) We invite every contributor to add themselves to the .zenodo.json file (https://github.com/nipreps/fmripost-template/blob/main/.zenodo.json), which will result in your being listed as an author at the next release. Please add yourself as the next-to-last entry, just above Russ.
   A pull request is a conversation. We may ask you to make some changes before accepting your PR, and likewise, you should feel free to ask us any questions you have.

[jhlegarreta]

I guess there is no way to tell that the appearance of this will be like the one expected until it it merged and another first time contributor walks in.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 26.36%. Comparing base (0448905) to head (c80f9b6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
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  Coverage   26.36%   26.36%           
=======================================
  Files          19       19           
  Lines        1358     1358           
  Branches      202      202           
=======================================
  Hits          358      358           
  Misses        990      990           
  Partials       10       10           

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[tsalo]

LGTM! Sorry I didn't notice this PR before now.

[jhlegarreta]

Feel free to merge @tsalo; I do not have merge rights in this repository.