deploy_site_github: Deploy a pkgdown site on Travis-CI to Github Pages
In Paradigm4/pkgdown: Make Static HTML Documentation for a Package

deploy_site_github() sets up your SSH keys for deployment, builds the site with build_site(), commits the site to the gh-pages branch and then pushes the results back to GitHub. deploy_site_github() is meant only to be used by the CI system on Travis, it should not be called locally. deploy_to_branch() can be used to deploy a site directly to GitHub Pages locally. See 'Setup' for details on setting up your repository to use this.

deploy_site_github(
  pkg = ".",
  install = TRUE,
  tarball = Sys.getenv("PKG_TARBALL", ""),
  ssh_id = Sys.getenv("id_rsa", ""),
  commit_message = construct_commit_message(pkg),
  clean = FALSE,
  verbose = FALSE,
  host = "github.com",
  ...,
  repo_slug = Sys.getenv("TRAVIS_REPO_SLUG", "")
)

`pkg`	Path to package.
`install`	Optionally, opt-out of automatic installation. This is necessary if the package you're documenting is a dependency of pkgdown
`tarball`	The location of the built package tarball. The default Travis configuration for R packages sets `PKG_TARBALL` to this path.
`ssh_id`	The private id to use, a base64 encoded content of the private pem file. This should not be your personal private key. Instead create a new keypair specifically for deploying the site. The easiest way is to use `travis::use_travis_deploy()`.
`commit_message`	The commit message to be used for the commit.
`clean`	Clean all files from old site.
`verbose`	Print verbose output
`host`	The GitHub host url.
`...`	Additional arguments passed to `build_site()`.
`repo_slug`	The `user/repo` slug for the repository.

For a quick setup, you can use usethis::use_pkgdown_travis(). It will help you with the following detailed steps.

Add the following to your .travis.yml file.

before_cache: Rscript -e 'remotes::install_cran("pkgdown")'
deploy:
  provider: script
  script: Rscript -e 'pkgdown::deploy_site_github()'
  skip_cleanup: true

Then you will need to setup your deployment keys. The easiest way is to call travis::use_travis_deploy(). This will generate and push the necessary keys to your GitHub and Travis accounts. See the travis package website for more details.
Next, make sure that a gh-pages branch exists. The simplest way to do so is to run the following git commands locally:
```
git checkout --orphan gh-pages
git rm -rf .
git commit --allow-empty -m 'Initial gh-pages commit'
git push origin gh-pages
git checkout master
```
We recommend doing this outside of RStudio (with the project closed) as from RStudio's perspective you end up deleting all the files and then re-creating them.
If you're using a custom CNAME, make sure you have set the url in _pkgdown.yaml:
```
url: http://pkgdown.r-lib.org
```