`get_extra_radiation` references could use an update

[kandersolar]

get_extra_radiation has five references:

pvlib-python/pvlib/irradiance.py

Lines 66 to 83 in b50b56f

	References
	----------
	.. [1] M. Reno, C. Hansen, and J. Stein, "Global Horizontal Irradiance
	Clear Sky Models: Implementation and Analysis", Sandia National
	Laboratories, SAND2012-2389, 2012.
	.. [2] http://solardata.uoregon.edu/SolarRadiationBasics.html, Eqs.
	SR1 and SR2
	.. [3] Partridge, G. W. and Platt, C. M. R. 1976. Radiative Processes
	in Meteorology and Climatology.
	.. [4] Duffie, J. A. and Beckman, W. A. 1991. Solar Engineering of
	Thermal Processes, 2nd edn. J. Wiley and Sons, New York.
	.. [5] ASCE, 2005. The ASCE Standardized Reference Evapotranspiration
	Equation, Environmental and Water Resources Institute of the American
	Civil Engineers, Ed. R. G. Allen et al.

There are a few issues:

• Ref 2 seems to not exist anymore. Here's an archive.org backup: https://web.archive.org/web/20240424224453/http://solardata.uoregon.edu/SolarRadiationBasics.html
• I am guessing the following reference should probably be added for method='spencer': J. W. Spencer, "Fourier series representation of the position of the sun" in Search 2 (5), p. 172 (1971)
• Duffie & Beckman could also be a relevant reference for method='spencer', but I don't think the 2nd edition (i.e., the cited edition) actually has it. I believe some later editions do.

[leliadeville]

The page for pvlib.atmosphere.get_relative_airmass specifies which citation corresponds to which specific model choice. It would be a nice addition when updating the references here.