Improve mdoc & markdown

Various small improvement on the mdoc and markdown side

more important stuff in no particular order:
* MDoc::original_mentity gives context
* README documents mgroups
* markdown need 4 spaces (not 5) for code-block and trims them
* most parts of test_markdown are refreshed

See examples in http://nitlanguage.org/code/test-markdown.html ; it just lists entities in an <h?> tag followed by its documentation