Theme-related Documentation Enhancements

- Consistent background color in all types of codes
- Improve dd (list item) padding
- Link to Index in global TOC in navbar
- Briefly describe Instaloader in `index.html` title
- Remove page contents (navbartoc) in index, genindex and installation
- Link to new pypi.org rather than old pypi.python.org

docs/_templates/globaltoc.html

@@ -0,0 +1,14 @@
+<li class="dropdown globaltoc-container">
+  <a role="button"
+     id="dLabelGlobalToc"
+     data-toggle="dropdown"
+     data-target="#"
+     href="{{ pathto(master_doc) }}">{{ theme_navbar_site_name }} <b class="caret"></b></a>
+  <ul class="dropdown-menu globaltoc"
+      role="menu"
+      aria-labelledby="dLabelGlobalToc">
+    {{ toctree(maxdepth=theme_globaltoc_depth|toint, collapse=False, includehidden=theme_globaltoc_includehidden|tobool) }}
+    <li role="separator" class="divider"></li>
+    <li class="toctree-l1"><a class="reference internal" href="genindex.html">Index</a></li>
+  </ul>
+</li>