Add documentation section on reverse transposition

[AdamRJensen]

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pvlib has two models for reverse transposition, i.e., estimating GHI from POA. The two models are currently listed in different parts of the documentation:

• DNI estimation models <- pvlib.irradiance.gti_dirint
• Transposition models <- irradiance.ghi_from_poa_driesse_2023

In this PR, I propose to add a new subsection under the Irradiance documentation section called "Reverse transposition".

In any case, it seems that these two models should be in the same part of the documentation.

Doc rendering link.