Generator expressions are supported in many places and are a distinct
concept worthy of their own manual page. The old builtin documentation
was previously represented by preprocessor macros to generate it into
each place that supports them. Factor out the duplicate content into a
dedicated cmake-generator-expressions manual page and reference it from
each original location.
Add a top-level document holding the Sphinx root toctree. List all
reference manuals from Help/manual/*.rst and the Sphinx-generated
general index (genindex) and search pages.
Brad King