added draft of the prevalence of malaria example (#691)

* added draft of the prevalence of malaria example

* added link to HSGP example, added lengthscale interpretation, added explanation to why we use matern vs expquad, added acknowledgements section under authors

* added coordinate reference system explanation and data processing narrative, made the graph after inference clearer, fixed typos

* replaced HSGP with GP in covariance functions talk

* updated priors on GP section

Author: Dekermanjian