Update contribution guidelines

[bouweandela]

The goal of this pull request is to align and clarify the documentation on contributing and the pull request checklist from the template.

TODO: replace https://esmvaltool--2031.org.readthedocs.build/en/2031/ with https://docs.esmvaltool.org/en/latest/ everywhere in the template before merging

• Link to documentation: https://esmvaltool--2031.org.readthedocs.build/en/2031/community/index.html

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• ☝ Create an issue to discuss what you are going to do

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• 🙏 We kindly ask you to review two other open pull requests in this repository

[nielsdrost]

Nice work @bouweandela!

Some comments on the table of content and section names:

• The introduction section perhaps needs a new name
• Release schedule could perhaps be after the pull request section, or at the end
• Moving work... section should be after the GitHub workflow (which explains there is a private repo)
• The 'Porting a namelist (recipe) or diagnostic from version 1 to version 2' section is perhaps a bit long for a title

[nielsdrost]

Awesome work @bouweandela! I read all the new documentation and checked as many links as I could, I think this is ready to go!