Doc-tests for files not generated by Modo

[mlange-42]

Like in the user guide, packages may want to display API docs and other documentation alongside each other on one site. As documetation that is not generated by Modo may still contain code examples, it would be useful to have a mechanism to extract doctests from them too.

Currently, this would only be possible by using dummy packages and/or modules to write the documentation.

Proposed solution

• Entire directories can be processed recursively (Allow a folder as input to process recursively #118).
• JSON files are processed as usual. Each JSON in the input directory (or sub-directories) becomes a package docs folder in the output (Allow a folder as input to process recursively #118).
• Markdown files are processed for doctests.