Replace chunk code with ad hoc function with trias function

[damianooldoni]

This pull request updates the documentation in cube-from-checklist.md to reflect changes in how users should retrieve GBIF backbone taxon keys for checklist datasets. The main change is the recommendation to use the get_nubkeys() function from the trias R package instead of providing a custom R function directly in the documentation.

Documentation update for checklist taxon key retrieval:

• Updated the instructions to recommend the get_nubkeys() function from the trias R package for retrieving GBIF backbone taxon keys, replacing the previously included custom R code and its explanation. The new guidance provides a direct link to the function's documentation and explains its usage and arguments.
• Removed the embedded R code example and related explanatory text for the custom name_backbone_gbif_checklist function, streamlining the tutorial and reducing maintenance burden.

[damianooldoni]

@peterdesmet, @sannegovaert: could you please review it? It will not take long. Thanks.
I cannot assign you as reviewers as it seems I made a PR from a fork under my GitHub account.