Make use of console/Rconsole fenced code blocks

[hturner]

We should use ```console fencing to markup input/output from the terminal (ref: https://stackoverflow.com/a/49004070) and similarly ```Rconsole fencing to mark up input/output from the R Console (ref: r-devel/r-dev-day#60).

Fenced code could replace some of the text only screenshots, making the docs easier to maintain and more accessible to users of screen readers and other assistive technologies

Marking (R) console output differently from code can help to differentiate code that should be copied and run vs input/output in the console that may be copied in whole/part, but is mainly for reference (ref: #106).