Add GHA's workflows, remove Travis and AppVeyor

[Enchufa2]

The main workflow, build.yml, builds and checks the package, and has the following properties:

• Triggered on push (all branches) and PRs.
• Adding [ci skip] to the head commit skips the build.
• Runs on macOS-latest-devel, macOS-latest-release, windows-latest-release, ubuntu-16.04-release, ubuntu-16.04-oldrel.
• Uploads artifacts on failure.
• Sends coverage to Codecov on success.
• Dispatches a build in {quantities} on success.

The secondary workflow, pkgdown.yml, builds and deploys the {pkgdown} site to gh-pages, and it's triggered only when a new release is published.

Also

• docs folder removed.
• Travis and AppVeyor configuration files removed.

[codecov[bot]]

Codecov Report

Merging #246 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #246   +/-   ##
=======================================
  Coverage   94.11%   94.11%           
=======================================
  Files          18       18           
  Lines         900      900           
=======================================
  Hits          847      847           
  Misses         53       53           

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