Add nav-bar screenshots (light + dark)

Navbar-only element capture at 1000px viewport from navbar-basic example.

Author: cderv

docs/websites/images/nav-bar-dark.png

@@ -34,7 +34,7 @@ website:
 
 Results in a navigation bar that looks something like this:
 
-![](images/nav-bar.png){fig-alt="A navigation bar. The title 'My Site' is on the left. To the right of the title are the words 'Home', 'Talks', and 'About'. 'Home' is slightly lighter than the other two words. On the far right side of the navigation bar is a search box."}
+![](images/nav-bar.png){.include-dark fig-alt="A navigation bar. The title 'My Site' is on the left. To the right of the title are the words 'Home', 'Talks', and 'About'. 'Home' is slightly lighter than the other two words. On the far right side of the navigation bar is a search box."}
 
 Above we use the `left` option to specify items for the left side of the navigation bar. You can also use the `right` option to specify items for the right side.