get gh pages workflow working

[billspat]

use github actions and workflow to build the book in to the ghpages branch to serve the book.

we have a workflow file but it's not being triggered , perhaps because it's triggered by a push only and not a merge and we've only merged into main?

on:
  push:
     branches:
       - master

nope that's not the problem

https://stackoverflow.com/questions/1783405/how-do-i-check-out-a-remote-git-branch

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#running-your-workflow-when-a-pull-request-merges

[billspat]

changes for this issue will be made in the master branch so we can test the workflow/action

[billspat]

for reference, found working workflows from Rstudio here: https://www.tidyverse.org/blog/2022/06/actions-2-0-0

[billspat]

currently crashes when attempting to use renv to install requirements - see #14