Outdated pvpmc.sandia.gov links in user guide.

[reepoi]

Describe the bug
The pvpmc.sandia.gov website was updated, breaking these links in the pvlib docs: the PV_LIB Matlab help webpage here, and PVPMC single diode page here.

[kandersolar]

@jsstein the PVLIB_MATLAB help pages don't seem to have gotten migrated; was that intentional?

For reference, here's an archive.org snapshot of those pages on the old site: http://web.archive.org/web/20230321205806/https://pvpmc.sandia.gov/PVLIB_Matlab_Help/