Update outdated pvpmc.sandia.gov links in user guide (#1789)

* update links

* Change matlab link to the repo page

Author: reepoi

docs/examples/iv-modeling/plot_singlediode.py

@@ -18,7 +18,7 @@
 # cell and has five electrical parameters that depend on the operating
 # conditions.  For more details on the single-diode equation and the five
 # parameters, see the `PVPMC single diode page
-# <https://pvpmc.sandia.gov/modeling-steps/2-dc-module-iv/diode-equivalent-circuit-models/>`_.
+# <https://pvpmc.sandia.gov/modeling-steps/2-dc-module-iv/single-diode-equivalent-circuit-models/>`_.
 #
 # References
 # ----------

docs/sphinx/source/user_guide/comparison_pvlib_matlab.rst

@@ -17,8 +17,8 @@ will require the efforts of the larger pvlib-python community, not just
 the maintainers. Therefore, do not expect feature parity between the
 libaries, only similarity.
 
-The `PV_LIB Matlab help webpage <https://pvpmc.sandia.gov/PVLIB_Matlab_Help/>`_
-is a good reference for this comparison.
+The `PV_LIB Matlab help pages <https://github.com/sandialabs/MATLAB_PV_LIB/>`_
+are a good reference for this comparison.
 
 Missing functions
 ~~~~~~~~~~~~~~~~~