v0.1.0rc3: Skel fixes (#46)

* add one to the heading level

This is because the text itself (e.g. Code des Impôts) should have level one

* split markdown_sekeleton so that it can be tested without a backend

* this separation does not get us more network-independance ; revert

* add (failing) test for extraneous notas

* make nota test pass

* refactor into function

* add test to exhibit nota inclusion

* nota format as per requested + test

* unused toc

* refactor

* appease mypy

* move to straight classes (dataclasses are meant to be mutable)

* see if explicitly specifying encoding helps with the windows tests...

* clean up html

* remove _clean_legifrance_html

* explicit newlines before nota