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.

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• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
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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
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If AI is used for any portion of this PR, you must vet the content
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[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.

[cwhanse]

Suggested change

	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). When backtracking is enabled, the rotation is
	adjusted away from this AOI-minimizing angle to avoid row-to-row shading.
	See [1]_ and [2]_ for details about the equations. Backtracking may be
	specified, in which case a ground coverage ratio is required.
	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). Backtracking may be specified, in which case
	a ground coverage ratio is required. When backtracking, the rotation is
	adjusted away from the AOI-minimizing angle to avoid row-to-row shading.
	See [1]_ and [2]_ for details about the equations.

[cwhanse]

Suggested change

	direct normal irradiance (DNI). (:issue:`2314`)
	direct normal irradiance (DNI). (:issue:`2314`) (:pull:`2869`)

[RDaxini]

Suggested change

	direct normal irradiance (DNI). (:issue:`2314`) (:pull:`2869`)
	direct normal irradiance (DNI). (:issue:`2314`, :pull:`2869`)