user guide: add warning when viewing markdown source file directly

doc/user_guide.md

@@ -24,6 +24,15 @@ custom build operations as well.
 
 Rscons is written in Ruby, and is inspired by [SCons](https://scons.org/) and [waf](https://waf.io/).
 
+${remove}
+WARNING: This user guide is meant to be preprocessed and rendered by a custom
+script.
+The markdown source file is not intended to be viewed directly and will not
+include all intended content.
+Visit [https://holtrop.github.io/rscons/index.html](https://holtrop.github.io/rscons/index.html)
+to view the properly rendered version.
+${/remove}
+
 ## Design Principles
 
 ### Build Correctness

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