update documentation

CONTRIBUTING.md

docs/source/conf.py

docs/source/guide/anomalies.rst

docs/source/guide/climatology.rst

docs/source/guide/contributing.rst

docs/source/guide/downloader.rst

docs/source/guide/modules.rst

docs/source/guide/quickstart.rst

docs/source/guide/smodex.rst

docs/source/index.rst