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/source_group.rst

@@ -0,0 +1,28 @@
+source_group
+------------
+
+Define a grouping for sources in the makefile.
+
+::
+
+  source_group(name [REGULAR_EXPRESSION regex] [FILES src1 src2 ...])
+
+Defines a group into which sources will be placed in project files.
+This is mainly used to setup file tabs in Visual Studio.  Any file
+whose name is listed or matches the regular expression will be placed
+in this group.  If a file matches multiple groups, the LAST group that
+explicitly lists the file will be favored, if any.  If no group
+explicitly lists the file, the LAST group whose regular expression
+matches the file will be favored.
+
+The name of the group may contain backslashes to specify subgroups:
+
+::
+
+  source_group(outer\\inner ...)
+
+For backwards compatibility, this command also supports the format:
+
+::
+
+  source_group(name regex)