Update instructions for releasing docs Gem (#6975)

Merge pull request 6975

docs/_docs/maintaining/releasing-a-new-version.md

@@ -69,6 +69,14 @@ And then, you're done! :tada: Feel free to celebrate!
 
 If you have access to the [@jekyllrb](https://twitter.com/jekyllrb) Twitter account, you should tweet the release post from there. If not, just ask another maintainer to do it or to give you access.
 
+### Build the docs
+
+We package our documentation as a :gem: Gem for offline use.
+
+This is done with the
+[**jekyll-docs**](https://github.com/jekyll/jekyll-docs#building) repository,
+and more detailed instructions are provided there.
+
 ## For non-core gems
 
 If you're not a maintainer for `jekyll/jekyll`, the procedure is much simpler in a lot of cases. Generally, the procedure still looks like this:

rake/docs.rake

@@ -1,25 +0,0 @@
-# frozen_string_literal: true
-
-#############################################################################
-#
-# Packaging tasks for jekyll-docs
-#
-#############################################################################
-
-namespace :docs do
-  desc "Release #{docs_name} v#{version}"
-  task :release => :build do
-    unless `git branch` =~ %r!^\* master$!
-      puts "You must be on the master branch to release!"
-      exit!
-    end
-    sh "gem push pkg/#{docs_name}-#{version}.gem"
-  end
-
-  desc "Build #{docs_name} v#{version} into pkg/"
-  task :build do
-    mkdir_p "pkg"
-    sh "gem build #{docs_name}.gemspec"
-    sh "mv #{docs_name}-#{version}.gem pkg"
-  end
-end