Move some testutils and overview docs to dev guide

Move some testutils and overview docs to dev guide

The unittesting.md page for Doxygen got stranded after the parent commit
that converted the dev-manual folder to Sphinx.  Make this page part of
the developer guide.  Move those parts that had extensive references to
C++ classes to the testutils module documentation page in Doxygen and
extend them a bit.

Also, move the codelayout.md to the developer guide, as it fits better
there and there are several crosslinks between it and the developer
guide, while there are very few from the Doxygen documentation.

Add a mechanism to easily create crosslinks from Doxygen to the
developer guide.

Add some overview text for the developer guide to explain the structure
of the developer documentation.

Some further reorganization is necessary (in child commits) to get
better organization for the text; for now, just converted the
formatting, without changing the contents or organization significantly
within the pages.

Change-Id: I68706ffc961e1c23f5de0d62118095e90e05f4dd