Clarify AOI-minimizing principle in tracking.singleaxis docstring

[chenyi6758-source]

The docstring for pvlib.tracking.singleaxis describes the coordinate system and rotation conventions in detail, but never states the underlying principle: the rotation angle is chosen to minimize the angle of incidence (AOI) of the sun's beam onto the panel, thereby maximizing the use of direct normal irradiance (DNI).

This PR adds that explanation to the docstring, including a note that backtracking adjusts the rotation away from the AOI-minimizing angle to avoid row-to-row shading. Also adds a whatsnew entry.

Closes #2314.

• I am familiar with the contributing guidelines
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• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
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[github-actions]

Hey @chenyi6758-source! 🎉

Thanks for opening your first pull request! We appreciate your
contribution. Please ensure you have reviewed and understood the
contributing guidelines.

If AI is used for any portion of this PR, you must vet the content
for technical accuracy.

[kandersolar]

@chenyi6758-source please edit the PR description to include the checklist from the PR template, and tick the items you are able to.

[chenyi6758-source]

Done — checklist added to the PR description, with the applicable items ticked.

[cwhanse]

@chenyi6758-source you can add your GH tag to the contributors at the bottom of the whatsnew file.