Start restructuring differentiating between language and rules

spec/changelog.tex

 \subsection*{Unreleased}
 
+Apply major changes to the structure of the document to clarify the difference
+between the \emph{language} and the \emph{rules}.
+
 List of rules:
 \begin{itemize}
     \item Improve description of $\proofRule{sko\_ex}$.