Updating 8 TeV links and documentation

[marianaiv]

I updated the links of the 8TeV notebooks to retrieve the data directly from the CERN Open Data portal. I also added documentation to the notebooks.

I suggest deleting:

• ATLAS_OpenData_8-TeV_cpp_simple_cut_and_count_analysis_example.ipynb: we have too many example notebooks on how to do plots and I don't think this one is necessary.
• ATLAS_OpenData_8-TeV_cpp_Z_mass_reconstruction_two_channels.ipynb or ATLAS_OpenData_8-TeV_cpp_Z_mass_reconstruction.ipynb since they are both doing the same analysis. I added documentation to both so we can choose which one to keep, or in case you want to keep both.

Closes MYATLAS-160

[Soap2G]

lgtm 👍