Convert CRAN links to preferred, canonical form

[concept-maps]: https://carpentries.github.io/instructor-training/05-memory/

[contrib-covenant]: https://contributor-covenant.org/

[contributing]: {{ site.github.repository_url }}/blob/gh-pages/CONTRIBUTING.md

[cran-checkpoint]: https://cran.r-project.org/web/packages/checkpoint/index.html

[cran-knitr]: https://cran.r-project.org/web/packages/knitr/index.html

[cran-stringr]: https://cran.r-project.org/web/packages/stringr/index.html

[cran-checkpoint]: https://cran.r-project.org/package=checkpoint

[cran-knitr]: https://cran.r-project.org/package=knitr

[cran-stringr]: https://cran.r-project.org/package=stringr

[email]: mailto:lessons@software-carpentry.org

[github-importer]: https://import.github.com/

[importer]: https://github.com/new/import