Add jekyll-redirect-from plugin in Gemfile

Created by: wangsai

Add jekyll-redirect-from plugin in Gemfile, ohterwise report

".../jekyll/plugin_mannager.rb:29:in require: cannot load such file -- jekyll-redirect-from (LoadError)"

when jekyll build

added docs v4 labels

changed milestone to %v4.0.0-alpha.2

closed