Improvements to vignettes

* All vignettes: set knit directory to working directory (probably: knitr::opts_knit$set(root.dir = getwd()) in setup chunk).
* Vignette "covariate": plot the pmf of the estimated Bernoulli by consistency with the first "getting started" vignette.
* Create new vignette "Bayesian Basics" reproducing existing teaching material with RSTooDs.