[FIX] doc: almost all warnings & drop mercurial dependency

* fix all warnings wrt patch application & al, only warning left is
  only exercise-kanban being auto-guessed as an xml+django file and
  then lexing failed (because it's only a small snippet)
* update sphinx-patchqueue to drop the requirement on mercurial for
  patch introspection and application, though that means application
  is significantly less forgiving (fuzzy application was not
  implemented)