docs: add contribution guidlines for extras

[begumcig]

Description

#622 #653

Related Issue

Fixes #(issue number)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactor (no functional change)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Testing

• I added or updated tests covering my changes
• Existing tests pass locally (uv run pytest -m "cpu and not slow")

For full setup and testing instructions, see the Contributing Guide.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code, especially for agent-assisted changes
• I updated the documentation where necessary

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[gsprochette]

I think the description here is still incomplete because it's missing the "how to add a new extra" part, and a "where to add an error to state that the extra should be installed to run this algorithm" part

[gsprochette]

I both agree with this, but this is not really for adding whatever test that requires an already defined extra, it's really for adding a new extra, correct?
I think this could be completed into a How and when to add a new extra section that explains

• how and when to create an extra in the pyproject.toml
• where to add an error in the algorithm to state that the extra should be installed
• how to mark the test and add the extra to the CI

[gsprochette]

Perfect, thanks for the changes 🚀