Site configuration for the gallery_site() generator.
gallery_site_config(input = ".")The function returns the contents of _site.yml as an R list, with
an additional element $gallery$meta, a list containing the metadata of
the pages to be generated, as read from the .json, .yml and yaml
files, where $gallery$type_field and gallery$type_template (if present)
have been already used to lookup the actual template. In addition,
default field values specified as gallery$defaults are also applied. The
metadata in $gallery$meta also include an additional field page_name
containing the names of the metadata list itself, serving as names for the
HTML pages that will be generated.