Add guidance about IAM to User Guide

[kandersolar]

• [ ] Closes #xxxx
• I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] 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`).
• [ ] 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.

Continuing in a broader thrust of developing our user guide section. Previously:

• Create weather data/iotools page in User's Guide #1754
• Add spectral mismatch model comparison table #2353
• Add guidance about SDMs to User Guide #2565
• Add guidance about temperature modeling to User Guide #2591

See also:

• Model comparison tables #2329

Docs build: https://pvlib-python--2683.org.readthedocs.build/en/2683/user_guide/modeling_topics/iam.html

[RDaxini]

Nice job @kandersolar , this is great. Thank you for putting this together! I left some comments on a few relatively minor things, but besides those few points this LGTM

[echedey-ls]

I'm glad to see pvlib incorporating these guides. I'm sure it will be very useful at many knowledge stages.

My two cents are:

• Maybe include at some point, even inside parentheses, a reference to circumsolar radiation usually being applied the direct IAM modifier.
• Note that IAM is also dependent on the wavelength, but characterization simplifies that since spectral performance modelling is infrequent and it's easier to characterize.

Thanks @kandersolar for hauling this initiative.

[kandersolar]

Note that IAM is also dependent on the wavelength, but characterization simplifies that since spectral performance modelling is infrequent and it's easier to characterize.

Good point. Reminder to add other missing aspects of these IAM models too:

• Polarization
• The light's "azimuth" angle, in the module's reference frame

[adriesse]

Good initiative!!