* doc/automake.texi (Top level): Rename as ...

* doc/automake.texi (Top level): Rename as ...
(Directories): ... this, and split into ...
(Subdirectories, Conditional Subdirectories): ... these.
(Conditional Subdirectories): Describe SUBDIRS and DIST_SUBDIRS
before the example.  Append a discussion about non-configured
conditional directories.
(Alternative): Move as a child of Directories.
(Subpackages): New section.
(Dist): Adjust links to Subdirectories, a Subpackages.
(Third-Party Makefiles): Link to Conditional Subdirectories.