[v9r3Docs: add Popularity documentation

This PR contains all the docs about the popularity, as well as the Excel spreadsheet needed for the analysis. it is not yet complete as I miss some info from @charpent

Also, for an easier review, please see: https://mylhcbdirac.readthedocs.io/en/v9r3_doc_popularity/AdministratorGuide/Popularity/index.html