[web] Added docs index page, fixed docs sidebar navigation.

This adds subsection support.

web/content/docs/_index.md

++++
+title = "Documentation"
+
+[[sections]]
+name = "Selected Benchmarks"
+slug = "benchmarks"
+text = "Basic benchmarks are explained and input files are provided to get you started in using OGS."
+
+[[sections]]
+name = "Developer Guide"
+slug = "devguide"
+text = "You want to contribute to the OpenGeoSys project or implement a custom feature? In the Developer Guide you will find everything to get you started into OpenGeoSys development. Experienced developers will find a comprehensive list of advanced development topics."
+
++++

web/layouts/docs/single.html

 {{ partial "header.html" . }}
   <div class="col-xs-3">
+    {{ $segments := split .RelPermalink "/" }}
+    {{ $subsection := index $segments 2 }}
     <ul>
     {{ range .Site.Pages.ByWeight }}
-      <!-- {{ if eq .Type "docs"}} -->
-        <li><a href="{{ .Permalink }}">{{ .Title }}</a></li>
-      <!-- {{ end }} -->
+      {{ if eq .Type "docs"}}
+        {{ $localSegments := split .RelPermalink "/" }}
+        {{ $localSubsection := index $localSegments 2 }}
+        {{ if eq $localSubsection $subsection  }}
+          <li><a href="{{ .Permalink }}">{{ .Title }}</a></li>
+        {{ end }}
+      {{ end }}
     {{ end }}
     <ul>
   </div>

web/layouts/section/docs.html

+{{ partial "header.html" . }}
+  <div class="col-xs-12">
+    <h1>OpenGeoSys Documentation</h1>
+    <div class="row">
+      {{ range .Params.sections }}
+        <div class="col-xs-4">
+          <h3><a href="/docs/{{ .slug }}">{{ .name }}</a></h3>
+          <p>{{ .text }}</p>
+        </div>
+
+      {{ end }}
+    </div>
+  </div>
+{{ partial "footer.html" . }}