start a "physics" section of the online docs

[tomeichlersmith]

there are a lot of topics that are helpful for sw developers and sw users to understand and share the same vocabulary, but they don't necessarily tie directly to the sw itself. For this reason, I think it would be helpful to have a separate section of the website devoted to these "physics" concepts (although some will just be statistical e.g. #25 ).

I already have some markdown notes on various things (some ecal-specific) that I could use as a starting point: https://github.com/tomeichlersmith/ldmx-ecal-validation/tree/main/characterize and tomeichlersmith/ldmx-ecal-validation#2