setting up gh-pages deploy

[maelle]

@karthik submitting this PR from master because I wanted to test things out in the fork. Cf https://plan.urssi.us/index.html

• I created an orphan gh-pages branch using https://gist.github.com/seanbuscay/5877413 (maybe I should remember how to do this one day...)

• I used the master.yml file of https://github.com/ropensci-books/http-testing because it has PDF and EPUB outputs too.

• I deleted a few files

• I added a GH Actions badge to the README.

If you create the gh-pages branch and merge this things should work. If not it serves as a better explanation than what I had said. :grin:

For ref https://github.com/r-lib/actions/tree/master/examples#build-bookdown-site

Log on my fork https://github.com/maelle/plan/runs/482315753?check_suite_focus=true

[maelle]

Having things deployed to a gh-pages branch breaks your preview integration :-(

Workflow for PRs in e.g. https://github.com/ropenscilabs/statistical-software-peer-review/blob/master/.github/workflows/pr.yml

• need two Netlify secrets (site ID, user auth) see https://github.com/ropenscilabs/statistical-software-peer-review/settings/secrets
• preview only when not from a fork -- so from a fork contrary to your current setup with the book as a subfolder, no preview. Pros: the contributor doesn't need to build the book. Contra: no preview for forks.