Temporarily add a RST formatter to convert builtin documentation to .rst
source files.  This will be removed shortly after we use it to convert
documentation.

Teach the RST formatter to:

* Output preformatted blocks as reStructuredText "::" literal blocks.

* Output option lists as bullet lists with option names enclosed in
  reStructuredText ``literal`` quoting.

* Output individual documentation objects (commands, variables, etc.)
  in separate .rst files organized in directories by type.

Replace references to cmVersion::GetCMakeVersion() in builtin
documentation with the literal placeholder "|release|" that will be
defined as a substitution later.