markdown: work on MDoc (instead of ADoc)

This commit broke the clients nitdoc, nitunit, and nitx.
The next commits will fix them (hopefully).

Signed-off-by: Jean Privat <jean@pryen.org>
nitdoc: adaptation for MDoc

The adaptation is minimal.

I tried to write a more complete adaptation but a in-depth rewriting
could be more useful since the new classes MEntity and MDoc should really
simplify things.

Signed-off-by: Jean Privat <jean@pryen.org>
nitunit: adapt do MDoc

Quite simple indeed.

Since the point is just to collect pieces of code, the walk on the AST is
still the best path.

Signed-off-by: Jean Privat <jean@pryen.org>
nitx: adapt to MDoc

all `modelbuilder.mthing2nthing` have disappeared.

However, the new MEntity class still need to be used to factorize code.

Signed-off-by: Jean Privat <jean@pryen.org>
test_markdown: adapt to MDoc

and update to the new lib/html API

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add and update test_markdown

A old version of sav/test_markdown.res existed but was not executed
by fulltest.sh

Signed-off-by: Jean Privat <jean@pryen.org>