Build docs via GitHub action running devtools::document() (on commit/PR)

[gregorgorjanc]

See https://github.com/AlphaGenes/AlphaPart/blob/devel/.github/workflows/document.yaml

[gregorgorjanc]

Hmm, is this actually working already? See 25d4937

[gregorgorjanc]

See

https://usethis.r-lib.org/reference/use_github_action.html

https://usethis.r-lib.org/reference/use_gitlab_ci.html