Add numpy ndarray.diagonal() documentation

[ylevental]

Description

Added comprehensive documentation for the NumPy ndarray.diagonal() method, which returns specified diagonal elements from an array.

This contribution includes:

• Complete method documentation with YAML front matter
• Detailed syntax section with parameter descriptions
• Three practical examples covering 2D arrays, rectangular matrices, and multi-dimensional arrays
• Interactive codebyte example demonstrating various diagonal extractions
• Clear explanations of how the method differs from .trace() and handles different array dimensions

File added: content/numpy/concepts/ndarray/terms/diagonal/diagonal.md

Issue Solved

Closes #7826

Type of Change

• Adding a new entry

Checklist

• All writings are my own.
• My entry follows the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked my entry and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• I have linked any issues that are relevant to this PR in the Issues Solved section.

[mamtawardhani]

Looks good for a second review! 🚀

[Sriparno08]

Looks good, @ylevental!

[github-actions]

👋 @ylevental
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