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/enable_testing.rst

@@ -0,0 +1,13 @@
+enable_testing
+--------------
+
+Enable testing for current directory and below.
+
+::
+
+  enable_testing()
+
+Enables testing for this directory and below.  See also the add_test
+command.  Note that ctest expects to find a test file in the build
+directory root.  Therefore, this command should be in the source
+directory root.