docs: Improve website + group articles + tweaks to documentation

[olivroy]

Follow-up to #902

• Improve function reference
• fix typos + links
• Correct cross-references based on the recent update
• Use tidyverse style guideline for references to functions. Enables linking on the website
• Move the raster to pkgdown/asset to allow users reproducing the example
• Move intro vignette to an article (there are many links + a widget which may cause problems on CRAN.
• Change headings to 1st and 2nd level when applicable.
• Reorder vignettes on the website
• Add leaflet.js copyright notice to the website
• Add @keywords internal for functions that are not necessarily user-facing to tidy up reference index

TODO: eventually remove vignettes/articles/nc after successful build.

[schloerke]

So sorry I didn't comment this earlier 🤦

[schloerke]

TODO: eventually remove vignettes/articles/nc after successful build.

If this is just for gh-pages, I'm happy to make an issue for a followup PR later. gh-pages isn't in the main branch, so it can have many copies of files

[schloerke]

Sorry @olivroy , you'll have to run devtools::document() to get rid of the failing docs check.

[schloerke]

⭐ Thank you @olivroy !! ⭐