Add infrastructure

[florianm]

User stories

• As a contributor, I want a zero-install developer experience I can launch in my browser, in addition to the "normal" local development experience (IDE like RStudio or VS Code).
• As a developer, I want maximal guidance on formalities.
• As a developer, I want to preview whether my PR will pass the CI tests.
• As a maintainer, I want to facilitate contributions.
• As a maintainer, I want to minimise the review burden. I.e., I don't want to review formalities, only load-bearing code changes.

Features

• Add a devcontainer setup to provide zero-install DX.
• Add a pkgdown setup to build package docs.
• Add GitHub Actions workflows to run CI (replacing TravisCI) and build docs (replacing readthedocs).
• Add test coverage reporting via codecov.io.
• Add pre-commit hooks to lint and format code. Leave invasive hooks commented out to split the plumbing (few additions) from the effects (most files reformatted).
• Add a contributing guide, update the README.