[FLINK-13723][docs] Use liquid-c for faster doc generation

Jekyll requires liquid and only optionally uses liquid-c if available. The
latter uses natively-compiled code and reduces generation time by ~5% for me.

This closes apache#9441

docs/Gemfile

@@ -27,6 +27,7 @@ gem 'therubyracer', '0.12.2'
 gem 'json', '2.0.4'
 gem 'jekyll-multiple-languages', '2.0.3'
 gem 'jekyll-paginate', '1.1.0'
+gem 'liquid-c', '4.0.0' # speed-up site generation
 
 group :jekyll_plugins do
  gem 'hawkins'

docs/Gemfile.lock

@@ -41,6 +41,8 @@ GEM
  kramdown (1.16.2)
  libv8 (3.16.14.19)
  liquid (4.0.0)
+ liquid-c (4.0.0)
+ liquid (>= 3.0.0)
  listen (3.1.5)
  rb-fsevent (~> 0.9, >= 0.9.4)
  rb-inotify (~> 0.9, >= 0.9.7)
@@ -80,11 +82,12 @@ DEPENDENCIES
  jekyll-multiple-languages (= 2.0.3)
  jekyll-paginate (= 1.1.0)
  json (= 2.0.4)
+ liquid-c (= 4.0.0)
  octokit (~> 4.3.0)
  therubyracer (= 0.12.2)
 
 RUBY VERSION
  ruby 2.3.1p112
 
 BUNDLED WITH
- 1.16.1
+ 1.17.2