[doc] add a script that appends Markdown and C++ files

bin/doc/merge_cpp_and_md.sh

+#!/bin/sh
+
+# The arguments should be names of Markdown and/or C++ files.
+# The script loops through all arguments. If a Markdown file is encountered,
+# its content is forwarded to stdout. For another file, a Markdown header
+# stating the filename is printed and the script cpp_to_md.sh is applied.
+
+for filename in "$@"
+do
+  if [[ ${filename: -3} == ".md" ]]; then
+    cat $filename
+  else
+    echo ""
+    echo ""
+    echo "## The file \`$filename\`"
+    echo ""
+    bash cpp_to_md.sh $filename
+    echo ""
+  fi
+done