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/math.rst

@@ -0,0 +1,13 @@
+math
+----
+
+Mathematical expressions.
+
+::
+
+  math(EXPR <output variable> <math expression>)
+
+EXPR evaluates mathematical expression and returns result in the
+output variable.  Example mathematical expression is '5 * ( 10 + 13
+)'.  Supported operators are + - * / % | & ^ ~ << >> * / %.  They have
+the same meaning as they do in C code.