Add tutorial about how to get GBIF checklist datasets via `rgbif`

Description

This tutorial is all about retrieving GBIF checklist datasets in R via rgbif. All other download functionalities are fully covered by the "new" rgbif vignettes provided by the new maintainer, John Waller.

Related Issue

This PR closes #210.

Task list

[x] My tutorial or article is placed in a subfolder of tutorials/content
[x] The filename of my tutorial or article is index.md. In case of an Rmarkdown tutorial I have knitted my index.Rmd to index.md (both files are pushed to the repo).
[x] I have included tags in the YAML header (see the tags listed in the tutorials website side bar for tags that have been used before)
[x] I have added categories to the YAML header and my category tags are from the list of category tags

Previewing the pull request

Thanks to GitHub Actions, an artifact (=zip file) of the rendered website is automatically created for each pull request. This provides a way to preview how these updates will look on the website, useful to contributors and reviewers.

Instructions to preview the updated website

1) On the PR page, you can find a "details" link under "checks - On PR, build the site and ...". Go there, click on the top link in the left sidebar ("Summary"), and download the generated artifact at the bottom of the page. 2) Decompress it and make sure the target directory is called 'tutorials' (you may need to rename it) 3) From the parent directory (just above the tutorials folder you created/renamed), run python -m http.server 8887, or launch the Google Chrome Web Server app and point it at the parent directory. 4) Point your browser to http://localhost:8887/tutorials. 5) Review the updated website. As a contributor, you can push extra commits to update the PR. As a reviewer, you can accept/refuse/comment the PR.

Note: for step 3, you can use any other simple HTTP server to serve the current directory if you don't have a Python 3 environment or Google Chrome available.

inbo / tutorials