Update contribution guidelines

[bouweandela]

Description

This pull request updates the contribution guidelines and the pull request template.

Closes #issue_number

Link to documentation: https://esmvaltool--1047.org.readthedocs.build/projects/ESMValCore/en/1047/contributing.html

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Before you get started

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

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🧪 The new functionality is relevant and scientifically sound
• 🛠 This pull request has a descriptive title and labels
• 🛠 Code is written according to the code quality guidelines
• 🧪 and 🛠 Documentation is available
• 🛠 Unit tests have been added
• 🛠 Changes are backward compatible
• 🛠 Any changed dependencies have been added or removed correctly
• 🛠 The list of authors is up to date
• 🛠 All checks below this pull request were successful

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To help with the number pull requests:

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

[nielsdrost]

Great work @bouweandela! One small comment (feel free to ignore) on a section with no intro or description, other than that good to go I think.

[bouweandela]

The scipy documentation appears to be offline, causing the documentation test on CircleCI to fail.