Remove Gemfile - use GitHub Pages default Jekyll setup

- Remove Gemfile and Gemfile.lock that were causing dependency conflicts
- Add supported plugins to _config.yml instead
- GitHub Pages will now use its default Jekyll version and dependencies
- This reverts to the working setup that existed before the Gemfile was added

Author: Phil Bennett

docs/Gemfile

@@ -20,6 +20,10 @@ highlighter: rouge
 
 permalink: pretty
 
+plugins:
+  - jekyll-feed
+  - jekyll-sitemap
+
 sass:
   style: compressed