[examples] Implement Python-only doc generator using pyparsing

Suggestion for a Python-only doc generator. It's using the library pyparsing. We used pyparsing in dune-testtools to parse input files and it's quite nice.

Supports:

• [[codeblock]] [[/codeblock]]
• [[exclude]] [[/exclude]]
• Excludes license header and header guards
• Removes extra lines at the end of code snippets

Should be easily extensible.

TODO

• Correct double spaces in some of the source files
• Get rid of doc above header guards
• Walk recursively through the folder structure to create README.mds in subfolders if .doc_config is present --> fixed in a different way: .doc_config can now specifiy multiple targets (markdown documentation files) which are generated by a list of source files. This way we can generate as many markdown files as we want, call them what we want, and generate them in the subfolder we want.