Add GHA that re-builds the pkgdown site after a PR is merged.

[cansavvy]

That's it, that's the whole idea.

A very rough version of what this would look like:

on: 
  push:
     branches: main 

jobs:
     <an environment that has pkgdown> 
     run: |
       Rscript -e "pkgdown::build_site()"
       git add docs/*
       git commit 
       git push main 

For reference, there are some other GHA here that we can borrow from: https://github.com/jhudsl/OTTR_Template/tree/main/.github/workflows

[howardbaik]

Here is a chapter on GHA that I should read: https://r-pkgs.org/software-development-practices.html#sec-sw-dev-practices-gha