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/_static/style.css

@@ -1,3 +1,12 @@
 code {
     color: #222;
-}
+    background-color: #f5f5f5;
+}
+
+dd {
+    padding-left: 8px;
+}
+
+dl.option dd {
+    padding-left: 12px;
+}