add jekyll-redirect-from gem

To be able to redirect to other sites not hosted within this repository
add/enable [jekyll-redirect-from] gem.

[jekyll-redirect-from]: https://github.com/jekyll/jekyll-redirect-from#redirect-to)

Author: agebhar1

_config.yml

@@ -29,3 +29,9 @@ collections:
   docs:
     output: true
     permalink: "/:title:output_ext"
+
+gems:
+  - jekyll-redirect-from
+
+whitelist:
+  - jekyll-redirect-from