[doc] example code for reading the default MD_DIMS

[pieleric]

The default value is explained in _metadata.py in English.
Explicitly give the equivalent Python version, as it's not obvious to
come up with it.

[coderabbitai]

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• doc/develop/backend-api.rst

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• doc/develop/backend-api.rst

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📝 Walkthrough

Walkthrough

This PR adds a documentation note to the backend API guide clarifying how to interpret MD_DIMS when it is undefined. It explains that the default dimension order should be derived from DataArray.ndim and gives the snippet: dims = data.metadata.get(model.MD_DIMS, "CTZYX"[-data.ndim::]).

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding example code documentation for reading the default MD_DIMS value.
Description check	✅ Passed	The description clearly explains the purpose of the change: providing an explicit Python code example for the default MD_DIMS logic that was previously only documented in English comments.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Pull request overview

This PR updates the developer backend API documentation to make the implied default MD_DIMS (“CTZYX” with leading dimensions dropped to match data.ndim) easier to apply by providing an explicit Python access pattern.

Changes:

• Added a .. note:: describing that MD_DIMS may be unset and how to derive the default order.
• Included a Python one-liner showing how to read MD_DIMS with a fallback derived from data.ndim.