Add Environments to side TOC in "Writing" (#121)

Author: gdalle

_layout/sidebar.html

@@ -24,6 +24,7 @@ <h1><a href="/">MoJuWo</a></h1>
           <li class="menu-list-item"><a href="/writing/#editor" class="menu-list-link">Editor</a>
           <li class="menu-list-item"><a href="/writing/#running_code" class="menu-list-link">Running code</a>
           <li class="menu-list-item"><a href="/writing/#notebooks" class="menu-list-link">Notebooks</a>
+          <li class="menu-list-item"><a href="/writing/#environments" class="menu-list-link">Environments</a>
           <li class="menu-list-item"><a href="/writing/#local_packages" class="menu-list-link">Local packages</a>
           <li class="menu-list-item"><a href="/writing/#development_workflow" class="menu-list-link">Development workflow</a>
           <li class="menu-list-item"><a href="/writing/#configuration" class="menu-list-link">Configuration</a>