Admin/page/main-layout/navigation.html

replaced some outdated HTML by more modern constructs

<div style="text-align: center; margin-top: 30px; margin-bottom: 45px">
<a href="index.html">
  <img src="isabelle.gif" width="100" height="86" alt="[Isabelle logo]" border="0">
</a>
</div>
<!-- _GP_ setnavcolor("#F0F0F0") -->
<!-- _GP_ page("Home", "index") -->
<!-- _GP_ empty_line(3) -->
<!-- _GP_ page("Overview", "overview") -->
<!-- _GP_ empty_line(3) -->
<!-- _GP_ page("Logics", "logics") -->
<!-- _GP_ empty_line(3) -->
<!-- _GP_ page("Documentation", "docs") -->
<!-- _GP_ empty_line(3) -->
<!-- _GP_ page("Download", "dist/index") -->
<!-- _GP_ empty_line(3) -->
&nbsp;