Support project folder outside of working directory

As filed by @TexKiller as cloudflare/doca#77

> If the project_folder is specified outside of the current directory, such as with `doca init -i ../schemas -o ../documentation`, the json schemas paths are messed up, ending up like `../../../schemas/schema.json`.
>
> The path to the schemas should be either absolute or relative to the project_folder, and not relative to the working directory.