Add NumPy ndarray .diagonal() method documentation

[Aggarwalmansi]

This PR introduces a new entry for the .diagonal() method under NumPy ndarray in content/numpy/concepts/ndarray/terms/diagonal/diagonal.md.

It includes:

A detailed explanation of the .diagonal() method and its use cases.

Syntax and parameter descriptions.

Examples demonstrating usage with 2D and 3D arrays, including diagonals with offsets and axis selection.

A compilable Codebyte example showing how to compute the trace of a matrix using .diagonal().

FAQs addressing common questions and clarifying differences from np.diag().
issue:#7826

[CLAassistant]

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2 out of 3 committers have signed the CLA.

✅ Aggarwalmansi
✅ mamtawardhani
❌ Mansi Agarwal

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[mamtawardhani]

@Aggarwalmansi please sign the CLA using your local ID also

[Aggarwalmansi]

I have signed it but its not updating , still i am trying multiple times but the page open as you have signed .Please check once and please merge if everything is fine

[mamtawardhani]

That's because its your local id. I would suggest raising another PR then

[mamtawardhani]

I'm closing this issue @Aggarwalmansi.

Also, please follow the protocols at Codecademy.
First, ask for the issue to be assigned to you, once the issue has been assigned, only then raise a PR for that issue.

Otherwise, we'll have to close the PR. Hope this helps!