[BUGFIX] Inconsistent naming of [cross_]axis_tilt / [cross_]axis_slope

[echedey-ls]

• Closes cross_axis_slope vs cross_axis_tilt #2334
• I am familiar with the contributing guidelines
• Tests added
• [NA] 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`).
• [NA] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

pvlib.shading.shaded_fraction1d uses 'cross_axis_slope' as an input, while pvlib.tracking.singleaxis uses 'cross_axis_tilt'. I think these are the same thing - is that correct? If so, would it make sense to update one of them to be consistent?

Extract from the body message, OP @williamhobbs, #2334 .

Also adds variable description to nomenclature page.

Relevant doc pages (old)

• https://pvlib-python--2543.org.readthedocs.build/en/2543/user_guide/extras/nomenclature.html#term-cross_axis_tilt
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.shading.shaded_fraction1d.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.tracking.singleaxis.html

Relevant doc pages (new)

• https://pvlib-python--2543.org.readthedocs.build/en/2543/whatsnew.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.shading.shaded_fraction1d.htm
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.shading.projected_solar_zenith_angle.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.tracking.singleaxis.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.tracking.calc_surface_orientation.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.tracking.calc_axis_slope.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.tracking.calc_cross_axis_slope.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/reference/generated/pvlib.pvsystem.SingleAxisTrackerMount.html
• https://pvlib-python--2543.org.readthedocs.build/en/2543/user_guide/extras/nomenclature.html#term-cross_axis_slope