fix documentation references to inverter/module data

[wholmgren]

The CEC data update in #767 broke the documentation. At least the ModelChain page is broken, maybe more.

[cwhanse]

Scanning the pages, these are also broken:
PVSystem attributes
PV Power Forecast

[birgits]

In the course of fixing this it would also be nice to fix the link to where the original files are downloaded from. I would have fixed it but could not find where the data sets are hosted.
Further, it would be great to provide a link to some documentation of the data sets in order for a user to figure out the meaning and units of the provided parameters. I think this link would be helpful:
https://sam.nrel.gov/photovoltaic/pv-sub-page-2.html

[cwhanse]

In the course of fixing this it would also be nice to fix the link to where the original files are downloaded from. I would have fixed it but could not find where the data sets are hosted.

Thanks for pointing out this old link. The current link.

Further, it would be great to provide a link to some documentation of the data sets in order for a user to figure out the meaning and units of the provided parameters. I think this link would be helpful:
https://sam.nrel.gov/photovoltaic/pv-sub-page-2.html

Agree, and again, thanks!