improve semiconductor plots

See merge request solidstate/lectures!113

This reverts commit 2804e237

Update 11_nearly_free_electron_model.md

See merge request solidstate/lectures!111

bold, italic text fix

Update 11_nearly_free_electron_model.md

See merge request solidstate/lectures!110

colors were changed in the figure, but not in the text referring to the figure

rework plots

See merge request solidstate/lectures!108

Update 14_doping_and_devices.md

See merge request solidstate/lectures!109

minor edit: replaced E_G with E_c ;

The exponent above it is not dependent on E_G. I think this is something that was used in previous years when E_G was the same as E_c 

Please check if this is correct before implementing