docs: update contributing guidelines

[JPXKQX]

Description

This PR updates the contributing guidelines with a new section about the scientific validation required for new features in Anemoi.

As a contributor to the Anemoi framework, please ensure that your changes include unit tests, updates to any affected dependencies and documentation, and have been tested in a parallel setting (i.e., with multiple GPUs). As a reviewer, you are also responsible for verifying these aspects and requesting changes if they are not adequately addressed. For guidelines about those please refer to https://anemoi.readthedocs.io/en/latest/

By opening this pull request, I affirm that all authors agree to the Contributor License Agreement.

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📚 Documentation preview 📚: https://anemoi--138.org.readthedocs.build/en/138/

[anaprietonem]

Thanks a lot Mario, to me the description for the label is clear and reads well!

[anaprietonem]

Suggested change

	other resources.
	other resources. Include profiling and benchmarking results comparing the implementation against a baseline run without the feature, using consistent hardware and settings.

[frazane]

This is reasonable, thanks for writing this down!

[mchantry]

Thanks Mario, at ATS today we decided to tweak the proposal slightly.
The idea would be that any PR (irrespective of ATS label) can receive this new label, and that ATS may decide in special circumstances to waive this requirement.
I can update the PR if you are happy.