build: add Gemfile with Jekyll 3.10 dependencies

Gemfile

@@ -0,0 +1,19 @@
+source "https://rubygems.org"
+
+# Use Jekyll 3.9 which is more stable and compatible with GitHub Pages
+gem "jekyll", "~> 3.9"
+gem "jekyll-feed", "~> 0.15"
+gem "jekyll-sitemap", "~> 1.4"
+gem "kramdown", "~> 2.3"
+gem "kramdown-parser-gfm", "~> 1.1"
+
+# Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
+# and associated library.
+platforms :mingw, :x64_mingw, :mswin, :jruby do
+  gem "tzinfo", ">= 1", "< 3"
+  gem "tzinfo-data"
+end
+
+# Performance and relevance
+gem "wdm", "~> 0.1.1", :platforms => [:mingw, :x64_mingw, :mswin]
+