Updates to the documentation.

docs/.static/css/custom.css

docs/data.png

docs/datamodel_dataflow.png

docs/datamodel_metadataflow.png

docs/datamodel_transformations.png

docs/introduction.md

docs/nomad.png

docs/parser_tutorial.md

gui/public/index.html

gui/src/components/Navigation.js

gui/src/pace.css

nomad/datamodel/__init__.py

nomad/files.py