Migrate continuous integration from Travis to GitHub Actions

Dropped integration with Travis CI, as we cannot further implement it for free (first problems were encountered at https://github.com/inbo/tutorials/pull/221#issuecomment-769633547). More background is provided in a post at rOpenSci: Moving away from Travis CI.

This PR implements three "GitHub Actions" workflows (tested in florisvdh/n2khab-fork):

R-CMD-check: modified from r-lib/actions/examples/check-standard.yaml.
- 6 jobs included (latest two Ubuntu LTS x {r-release, r-devel}; Windows with r-release; MacOS with r-release). Essentially performs R CMD check --as-cran --no-manual.
- Jobs are triggered:
- on push (all branches)
- on pull-requests to master (job is run on the would-be merge-commit)
- monthly (on master)
- Check results (zip) are saved as artifact per job (90 days available).
site-deploy: modified from r-lib/actions/examples/pkgdown.yaml.
- One job; builds pkgdown website and pushes it to gh-pages. Runs on Ubuntu 20.04.
- Triggered when new commit is added to master.
site-devel: modified from site-deploy.
- One job; builds pkgdown website without deploying. Runs on Ubuntu 20.04.
- Triggered on any pull-request (job is run on the would-be merge-commit).
- The rendered pkgdown website (zip) is saved as an artifact with the job (90 days available).

All workflows use the standard CRAN repository. All Linux workflows also use the ubuntugis-unstable PPA to get (more) recent geospatial libraries.

inbo / n2khab