Convert builtin help to reStructuredText source files

Run the convert-help.bash script to convert documentation:

 ./convert-help.bash "/path/to/CMake-build/bin"

Then remove it.

Help/command/use_mangled_mesa.rst

@@ -0,0 +1,13 @@
+use_mangled_mesa
+----------------
+
+Copy mesa headers for use in combination with system GL.
+
+::
+
+  use_mangled_mesa(PATH_TO_MESA OUTPUT_DIRECTORY)
+
+The path to mesa includes, should contain gl_mangle.h.  The mesa
+headers are copied to the specified output directory.  This allows
+mangled mesa headers to override other GL headers by being added to
+the include directory path earlier.