Allow for YAML files to be commented, etc

[aidansteele]

Right now we obliterate the contents of the file on each generation. Instead we should parse the files and determine which records are already in there. Then we can append the new entries. This way we can add human comments to the files or split up big ones, etc.

Caveat: when moving an entry from the "actions without APIs" or "APIs without actions" lists to the sane list, that involves reordering - probably want to bring comments along for the ride somehow 🤔