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

[Jayclifford345]

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.

[jdbaldry]

Out of interest, what does Killercoda do if we specify a file that doesn't exist. Do they still render a page that's blank?