feat: Tool to check validity of files found in details section

If I specify a finish markdown file (instead of using the finish directive) maybe check that file exists, otherwise in my markdown metadata, I can promise you that a file is there which won't be there, which will result in a rendered broken guide. I shouldn't be able to lie about the existence of files.

Start by locating the Go code that parses markdown metadata and handles the finish directive or finish file. Trace how referenced files are resolved, then add validation so a missing finish file is reported before rendering; done means invalid metadata cannot produce a guide with a broken finish-file reference.

Written by the indexing model from the issue text.