Address rOpenSci review requests

[tklebel]

Review at: https://github.com/ropensci/onboarding/issues/189

Review by jsonbecker

• [x] change basename_id to file_name.
• [x] explain why file_name is a good identifier (seconded by elinw)
• [x] think about export option to sqlite
• [x] add import function to read csvs from jstor_import #40
• [x] clarify, whether find_authors works on books or not
• [x] check, if find_footnotes and find_references could be used for books too
• [x] fix vignette formatting
• [x] improve error messages for find_references to say, that references are not available for books

Review by elinw

• [x] add vignette with "known quirks" and how to handle them
• [x] check aspect with repeated endnotes