Add Documentation Dangerfile

Every time a doc change is made, a comment to
ping the @gl-docsteam is added.

Dangerfile

@@ -4,4 +4,5 @@ danger.import_dangerfile(path: 'danger/changelog')
 danger.import_dangerfile(path: 'danger/specs')
 danger.import_dangerfile(path: 'danger/gemfile')
 danger.import_dangerfile(path: 'danger/database')
+danger.import_dangerfile(path: 'danger/documentation')
 danger.import_dangerfile(path: 'danger/frozen_string')

danger/documentation/Dangerfile

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+# All the files/directories that should be reviewed by the Docs team.
+DOCS_FILES = [
+  'doc/'
+].freeze
+
+def docs_paths_requiring_review(files)
+  files.select do |file|
+    DOCS_FILES.any? { |pattern| file.start_with?(pattern) }
+  end
+end
+
+all_files = git.added_files + git.modified_files
+
+docs_paths_to_review = docs_paths_requiring_review(all_files)
+
+unless docs_paths_to_review.empty?
+  message 'This merge request adds or changes files that require a ' \
+    'review from the docs team.'
+
+  markdown(<<~MARKDOWN)
+## Docs Review
+
+The following files require a review from the Documentation team:
+
+* #{docs_paths_to_review.map { |path| "`#{path}`" }.join("\n* ")}
+
+To make sure these changes are reviewed, mention `@gl-docsteam` in a separate
+comment, and explain what needs to be reviewed by the team. Please don't mention
+the team until your changes are ready for review.
+  MARKDOWN
+
+  unless gitlab.mr_labels.include?('Documentation')
+    warn 'This merge request is missing the ~Documentation label.'
+  end
+end

doc/development/documentation/index.md

@@ -257,6 +257,15 @@ choices:
 If your branch name matches any of the above, it will run only the docs
 tests. If it doesn't, the whole test suite will run (including docs).
 
+## Danger bot
+
+GitLab uses [danger bot](https://github.com/danger/danger) for some elements in
+code review. For docs changes in merge requests, the following actions are taken:
+
+1. Whenever a change under `/doc` is made, the bot leaves a comment for the
+   author to mention `@gl-docsteam`, so that the docs can be properly
+   reviewed.
+
 ## Merge requests for GitLab documentation
 
 Before getting started, make sure you read the introductory section