Replace some 'error' instances with 'user-error'
Manually export README for ox-md changes

Seems like ox-md changed how it handles spacing.
Mention Org-ref in README
bog-rename-staged-file-to-citekey: Fix docstring
bog-root-directory: Simplify docstring
bog-selection-method: Add docstring
Use cache for bog-all{-heading}-citekeys
Add command bog-insert-heading-citekey