docs: Provide bundler setup for building site locally

This mimics the GitHub Pages environment so that you can build and serve
the site locally for testing. It's will also be required later for using
Jekyll Actions[1] instead of the automated GitHub Pages flow.

1. https://github.com/marketplace/actions/jekyll-actions

docs/Gemfile

@@ -0,0 +1,14 @@
+# Bundler setup for jekyll to be deployed on github pages.
+
+source "https://rubygems.org"
+
+# Note that we're using the github-pages gem to mimic the GitHub pages
+# automated setup. That installs jekyll, a default set of jekyll
+# plugins, and a modified jekyll configuration.
+group :jekyll_plugins do
+  gem "github-pages"
+  gem "jekyll-remote-theme"
+end
+
+# Prefer the GitHub flavored markdown version of kramdown.
+gem "kramdown-parser-gfm"