Improve the `doi()` help, to include a working example.

[SimonGoring]

In response to an article review, it's clear that we need to improve the documentation for the doi() function. I have written a GitHub Gist that effectively does what we're looking to do.

We should clean up the code so it comes directly from a set of queries (rather than a raw file):

library(neotoma2)
library(dplyr)
library(readr)

poll <- readr::read_tsv('pollen_counts_europe.csv') %>%
    dplyr::filter(Data_Source == 'Neotoma') %>%
    dplyr::select(Dataset_ID) %>%
    dplyr::distinct() %>%
    unlist()

datasets <- neotoma2::get_datasets(poll, all_data = TRUE)
neotoma2::doi(datasets)