R: Vignettes and other documentation

# Vignettes and other documentation

## Vignettes from package 'testthat'

testthat::custom-expectation |  | Custom expectations | HTML | source | R code
testthat::parallel |  | Running tests in parallel | HTML | source | R code
testthat::skipping |  | Skipping tests | HTML | source | R code
testthat::snapshotting |  | Snapshot tests | HTML | source | R code
testthat::test-fixtures |  | Test fixtures | HTML | source | R code
testthat::third-edition |  | testthat 3e | HTML | source | R code