[FLINK-13726][docs] Build docs with jekyll 4.0.0.pre.beta1

This significantly reduces the build times, on my machine from 140s to 47s!

This closes apache#9444

docs/.gitignore

@@ -1,6 +1,7 @@
 .bundle/
 .jekyll-metadata
+.jekyll-cache/
 .rubydeps/
 content/
 ruby2/.bundle/
-ruby2/.rubydeps/
+ruby2/.rubydeps/

docs/Gemfile

@@ -18,9 +18,9 @@
 
 source 'https://rubygems.org'
 
-ruby '>= 2.1.0'
+ruby '>= 2.4.0'
 
-gem 'jekyll', '3.7.2'
+gem 'jekyll', '4.0.0.pre.beta1'
 gem 'addressable', '2.6.0'
 gem 'octokit', '4.14.0'
 gem 'therubyracer', '0.12.3'

docs/Gemfile.lock

@@ -14,20 +14,21 @@ GEM
     ffi (1.11.1)
     forwardable-extended (2.6.0)
     http_parser.rb (0.6.0)
-    i18n (0.9.5)
+    i18n (1.6.0)
       concurrent-ruby (~> 1.0)
-    jekyll (3.7.2)
+    jekyll (4.0.0.pre.beta1)
       addressable (~> 2.4)
       colorator (~> 1.0)
       em-websocket (~> 0.5)
-      i18n (~> 0.7)
+      i18n (>= 0.9.5, < 2)
       jekyll-sass-converter (~> 1.0)
       jekyll-watch (~> 2.0)
-      kramdown (~> 1.14)
+      kramdown (~> 2.1)
+      kramdown-parser-gfm (~> 1.0)
       liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
-      rouge (>= 1.7, < 4)
+      rouge (~> 3.0)
       safe_yaml (~> 1.0)
     jekyll-multiple-languages (2.0.3)
     jekyll-paginate (1.1.0)
@@ -36,7 +37,9 @@ GEM
     jekyll-watch (2.2.1)
       listen (~> 3.0)
     json (2.2.0)
-    kramdown (1.17.0)
+    kramdown (2.1.0)
+    kramdown-parser-gfm (1.1.0)
+      kramdown (~> 2.0)
     libv8 (3.16.14.19)
     liquid (4.0.3)
     liquid-c (4.0.0)
@@ -80,7 +83,7 @@ PLATFORMS
 
 DEPENDENCIES
   addressable (= 2.6.0)
-  jekyll (= 3.7.2)
+  jekyll (= 4.0.0.pre.beta1)
   jekyll-multiple-languages (= 2.0.3)
   jekyll-paginate (= 1.1.0)
   json (= 2.2.0)
@@ -90,7 +93,7 @@ DEPENDENCIES
   therubyracer (= 0.12.3)
 
 RUBY VERSION
-   ruby 2.3.1p112
+   ruby 2.6.3p62
 
 BUNDLED WITH
    1.17.2

docs/README.md

@@ -42,8 +42,7 @@ If you call the script with the preview flag `build_docs.sh -p`, Jekyll will
 start a web server at `localhost:4000` and watch the docs directory for
 updates. Use this mode to preview changes locally. 
 
-If you have ruby 2.0 or greater, 
-you can call the script with the incremental flag `build_docs.sh -i`.
+You can call the script with the incremental flag `build_docs.sh -i`.
 Jekyll will then serve a live preview at `localhost:4000`,
 and it will be much faster because it will only rebuild the pages corresponding
 to files that are modified. Note that if you are making changes that affect