doc-rst: make dev-tools folder buildable stand-alone

Add minimal conf.py and moved dev-tools/tools.rst to dev-tools/index.rst
makes the dev-tools folder buildable stand-alone. To build only this
folder run::

  make SPHINXDIRS=dev-tools htmldocs
  make SPHINXDIRS=dev-tools pdfdocs

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>