Revert memoizing Site#docs_to_write and #documents (#7684)

Merge pull request 7684

lib/jekyll/site.rb

@@ -326,15 +326,15 @@ module Jekyll
     #
     # Returns an Array of Documents which should be written
     def docs_to_write
-      @docs_to_write ||= documents.select(&:write?)
+      documents.select(&:write?)
     end
 
     # Get all the documents
     #
     # Returns an Array of all Documents
     def documents
-      @documents ||= collections.reduce(Set.new) do |docs, (_, collection)|
-        docs + collection.docs + collection.files
+      collections.each_with_object(Set.new) do |(_, collection), set|
+        set.merge(collection.docs).merge(collection.files)
       end.to_a
     end