Automatically build the documentation using GitHub actions

[berntpopp]

Configure automatic builds of the documentation using GitHub actions.

Examples:

• https://medium.com/@delucmat/how-to-publish-bookdown-projects-with-github-actions-on-github-pages-6e6aecc7331e
• https://github.com/rstudio/bookdown/actions
• https://lucacozzuto.medium.com/automatic-reports-with-bookdown-and-github-actions-3c19498394aa

[berntpopp]

This relates to #5