docs: Docs minor fixes update concat yaml

[faemetm]

Description

Update the concat.yaml documentation to use the new dataset-based syntax

What problem does this change solve?

This change updates outdated documentation. It is a doc update, not a code change. The old example could be confusing for users following the Anemoi framework, so the new example reflects the correct current syntax. The syntax has been confirmed to work with the dataset argument. Its compatibility with other sources has not yet been tested.

What issue or task does this change relate to?

No linked issue; this is a documentation improvement for clarity.

Additional notes

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-datasets--539.org.readthedocs.build/en/539/

[floriankrb]

This is a very good idea to ifx the documentation, thank you.

This could be merged as soon as the branch conflicts have been resolved.