Help: Add cmake-buildsystem.7 manual

Describe how to define a buildsystem of binary targets, how to
express dependencies between them, how to add build specifications,
how to specify usage requirements, transitive and compatible
propagation and the various pseudo targets.

Help/index.rst

@@ -25,6 +25,7 @@ Reference Manuals
 .. toctree::
    :maxdepth: 1
 
+   /manual/cmake-buildsystem.7
    /manual/cmake-commands.7
    /manual/cmake-developer.7
    /manual/cmake-generator-expressions.7