user guide: add warning when viewing markdown source file directly

rb/gen_user_guide.rb

@@ -59,6 +59,8 @@ class Generator
       line = @lines.slice!(0)
       if line =~ /^```(.*)$/
         @sections.last.append(gather_code_section($1))
+      elsif line.chomp == "${remove}"
+        remove_section
       elsif line =~ /^(#+)(>)?\s*(.*)$/
         level_text, new_page_text, title_text = $1, $2, $3
         level = $1.size
@@ -196,6 +198,15 @@ class Generator
     end
   end
 
+  def remove_section
+    loop do
+      line = @lines.slice!(0)
+      if line.chomp == "${/remove}"
+        break
+      end
+    end
+  end
+
   def get_next_section_number(level)
     if @current_section_number.size == level - 1
       @current_section_number << 1