Added documentation on how to access data directly through the API

Added a quick overview that covers:

How to use the /datasets endpoint to explore available datasets
How to use the /datasets/{spotlightId} endpoint to filter datasets by spotlight
How to use the dataset metadata (timeUnit, isPeriodic etc) to correctly fill out the source.tile URL to access data
How to display/download data from a correctly formed URL

NASA-IMPACT / covid-api