Add header link to GitHub project source code on docs site.

Author: geerlingguy

docs/_config.yml

@@ -25,6 +25,9 @@ markdown: kramdown
 theme: minima
 plugins: []
 
+header_pages:
+  - index.md
+
 # Exclude from processing.
 # The following items will not be processed, by default. Create a custom list
 # to override the default setting.