Export a project's grading marks to YAML
marks_export.RdWrites the per-repository grading state stored in the project's grading
database (each repository/question pair's selected rubric items, identified
by description, plus public and private comments) to a YAML file. Only
pairs with grading activity are written; a pair whose items were all
deselected exports as items: []. The file can be edited and brought back
with marks_import(), and its format is described by the JSON Schema at
system.file("schema/markermd-marks.json", package = "markermd").