ADd warning for outdated versioned content

Rules

@@ -54,6 +54,7 @@ compile '*' do
   elsif item[:extension] == 'md'
     filter :redcarpet, options: {filter_html: true, autolink: true, no_intraemphasis: true, fenced_code_blocks: true, gh_blockcode: true, tables: true}, renderer_options: {with_toc_data: true}
     filter :normalize_links, item[:repo_docs] if item[:repo_docs]
+    filter :outdated_content, item[:repo_docs] if item[:repo_docs] && item[:repo_docs][:outdated]
     filter :add_anchors
     filter :bootstrappify
     filter :admonition

lib/filters/outdated_content.rb

+# encoding: utf-8
+
+require 'nokogiri'
+
+class OutdatedContent < ::Nanoc::Filter
+  identifier :outdated_content
+
+  def run(content, params = {})
+    doc = Nokogiri::HTML(content)
+    # TODO(ts): We need to link to the same page or the first child without hardcoding /getting_started/.
+    warning = %(<p>CAUTION: This page documents an old version of Prometheus.
+      Check out the <a href="#{params[:outdated]}getting_started/">latest version</a>.</p>)
+
+    body = doc.css('body')
+    if first = body.children.first
+      first.add_previous_sibling(warning)
+    else
+      body << Nokogiri::HTML::DocumentFragment.parse(warning)
+    end
+
+    doc.to_s
+  end
+end

nanoc.yaml

@@ -82,6 +82,7 @@ data_sources:
       name: '1.8'
       repository: https://github.com/prometheus/prometheus.git
       refspec: release-1.8
+      outdated: /docs/prometheus/latest/
   -
     type: static
     items_root: /assets/