More packaging documentation.

lib/spack/docs/_themes/sphinx_rtd_theme/footer.html

lib/spack/docs/conf.py

lib/spack/docs/developer_guide.rst

lib/spack/docs/packaging_guide.rst

lib/spack/spack/cmd/__init__.py

lib/spack/spack/cmd/checksum.py

lib/spack/spack/cmd/create.py

lib/spack/spack/cmd/edit.py

lib/spack/spack/package.py

lib/spack/spack/packages/libelf.py