Extract descriptive metadata from docstrings

[kba]

In addition to or replacing the ocrd-tool.json fields or README, it might make sense to encourage proper python documentation with docstrings on modul level at least for the processors and use that information for the generation of documentation.

[bertsky]

I am not sure we need this as part of the specs anymore. It has been implemented in core already, but since this applies to internal of Python implementations, I don't think it should be described here.

Some of the proposals in https://github.com/OCR-D/core/issues/537 are more far-reaching than docstrings, though – dumping READMEs, repo URLs, license information on request.