Update cams links

[AdamRJensen]

• Closes Update reference in pvlib.iotools.read_cams #1515
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  - [ ] Tests added
  - [ ] Updates entries in docs/sphinx/source/reference for API changes.
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This PR updates the links to the documentation for CAMS radiation (see #1515)

CAMS has requested that the following link be used: https://atmosphere.copernicus.eu/solar-radiation. The two previous links to SoDa's sites for CAMS Radiation and McClear have been removed, although one link to the SoDa remains as it describes the API used by the get_cams function.

Here's an explanation from Marion Schroedter Homscheidt:

ECMWF now made a link to the confluence page (which is the new master documentation source) from the overview page https://atmosphere.copernicus.eu/solar-radiation . . . from there users will find the master documentation and if they need more details the full user guide document (which is linked there as pdf). Please note that there is no separate documentation anymore for CAMS Radiation and CAMS McClear services. The clear-sky model is part of the CAMS Radiation service. So, both links should direct to https://atmosphere.copernicus.eu/solar-radiation. The SoDa pages won’t be deleted as SoDa users may be used to them. But they will be phased out gradually and will in future link also to the CAMS confluence pages.

[wholmgren]

Thanks @AdamRJensen. I hope the SodaPro API remains available so that I don't have to bother with cdsapi. Feel free to merge tomorrow unless someone objects.

[AdamRJensen]

@kanderso-nrel I don't understand why the coverage is failing.. do you have a hint?

[kandersolar]

Not entirely sure, but it's spurious here of course. Maybe updating w/ the latest master would fix it. In any case fine to ignore it.