ADD link to tutorial for matplotlib plotting

[skloibhofer]

Just like in the docs page for plotly, I added the link to the tutorial Plotting Pandapower Networks to the documentation of plotting/matplotlib/simple_plot.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cedb56f) 79.65% compared to head (766d4ed) 79.62%.
Report is 1 commits behind head on develop.

❗ Current head 766d4ed differs from pull request most recent head 3a0763a. Consider uploading reports for the commit 3a0763a to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2132      +/-   ##
===========================================
- Coverage    79.65%   79.62%   -0.03%     
===========================================
  Files          255      255              
  Lines        27795    27798       +3     
===========================================
- Hits         22139    22134       -5     
- Misses        5656     5664       +8     

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