NeXus-related documentation in NOMAD

docs/examples/experiment_data/apm.md

docs/examples/experiment_data/em.md

docs/examples/experiment_data/mpes.md

docs/examples/experiment_data/nexus.md

docs/examples/experiment_data/opt.md

docs/examples/experiment_data/pynxtools.md

docs/examples/experiment_data/sts.md

docs/examples/experiment_data/xps.md

docs/examples/overview.md

gui/tests/env.js

mkdocs.yml