Provide definitions for obscure terms

[lucasmarohn]

In areas where more obscure terms are used, we should provide a clear definition.

Below the tabs and above the graphs, we could display a definition. Cases and Deaths are obvious, but Mortality should definitely be defined. We could also link to Methodology from here.

Cases: An instance of the COVID-19 viral infection

Deaths: Deaths caused by COVID-19

Mortality: Deaths expressed as a proportion of the population

Example page: https://virustrack.live/covid