Support external navigation links (#876)

Based on #238, resolves #66.

docs/configuration.md

@@ -106,6 +106,11 @@ aux_links_new_tab: false
 heading_anchors: true
 ```
 
+## External navigation links
+
+External links can be added to the navigation through the `nav_external_links` option.
+See [Navigation Structure]({{ site.baseurl }}{% link docs/navigation-structure.md %}#external-navigation-links) for more details.
+
 ## Footer content
 
 ```yaml

docs/navigation-structure.md

@@ -246,6 +246,25 @@ aux_links:
 
 ---
 
+## External Navigation Links
+
+To add external links to the navigation, add them to the `nav_external_links` [configuration]({{ site.baseurl }}{% link docs/configuration.md %}) option in your site's `_config.yml` file.
+External links will appear under all other items in the listing order.
+
+#### Example
+
+{: .no_toc }
+
+```yaml
+# External navigation links
+nav_external_links:
+  - title: Just the Docs on GitHub
+    url: https://github.com/just-the-docs/just-the-docs
+    hide_icon: false # set to true to hide the external link icon - defaults to false
+```
+
+---
+
 ## In-page navigation with Table of Contents
 
 To generate a Table of Contents on your docs pages, you can use the `{:toc}` method from Kramdown, immediately after an `<ol>` in Markdown. This will automatically generate an ordered list of anchor links to various sections of the page based on headings and heading levels. There may be occasions where you're using a heading and you don't want it to show up in the TOC, so to skip a particular heading use the `{: .no_toc }` CSS class.