Update irradiance.haydavies docs; use Wm⁻² instead of W/m^2 throughout pvlib.irradiance (#2191)

* Update irradiance.py

add eqn variable definitions
rearrange references
changed beta into theta_T to avoid double definition --- surface tilt, and "daily average of the solar elevation in degrees" (pvlib.irradiance.diffuse_par_spitters)

* Update irradiance.py

* Update irradiance.py

Remove Loutzenhiser P.G. et. al. reference. I do not see it cited anywhere here in the docs for the haydavies model

* Update irradiance.py

add "projection ratio" to variable definition

* Update irradiance.py

separate A and R_b

* Update irradiance.py

Change all instances of W/m^2 into Wm⁻²

* Update irradiance.py

W/m^2 to Wm⁻²

* Update irradiance.py

* add url to references

* Update irradiance.py

* unit typesetting suggestion

* Update irradiance.py

* Update irradiance.py

* Update pvlib/irradiance.py

Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>

* eqn typesetting

* minor wording edits

* Update v0.11.1.rst

* Update irradiance.py

adjust units

Co-Authored-By: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>

* Update irradiance.py

* \theta_T -> \beta

* add last accessed to reference

* remove spurious period

* Apply suggestions from code review

Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>

* Update irradiance.py

Co-Authored-By: Cliff Hansen <5393711+cwhanse@users.noreply.github.com>

* Update irradiance.py

---------

Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>
Co-authored-by: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>
Co-authored-by: Cliff Hansen <5393711+cwhanse@users.noreply.github.com>

Author: 4 people

docs/sphinx/source/whatsnew/v0.11.1.rst

@@ -62,16 +62,16 @@ Documentation
 * Added gallery example demonstrating the application of
   several spectral mismatch factor models.
   (:issue:`2107`, :pull:`2114`)
-
 * Added gallery example on calculating cell temperature for
   floating PV. (:pull:`2110`)
-
 * Added gallery example demonstrating how to use
   different Perez coefficients in a ModelChain.
   (:issue:`2127`, :pull:`2148`)
 * Fixed examples in :py:func:`pvlib.shading.shaded_fraction1d`. (:pull:`#2215`)
 
 * Removed unused "times" input from dni_et() function (:issue:`2105`)
+* Updated :py:func:`pvlib.irradiance.haydavies` to include equation variable
+  definitions and a new "notes" section (:issue:`2183`, :pull:`2191`)
 
 * Split the contributing page into several pages (:issue:`2210`, :pull:`2219`)