Resolve "Improved plugin documentation"

docs/explanation/plugin_system.md

docs/howto/overview.md

docs/howto/plugins/apis.md

docs/howto/plugins/apps.md

docs/howto/plugins/example_uploads.md

docs/howto/plugins/normalizers.md

docs/howto/plugins/parsers.md

docs/howto/plugins/plugins.md

docs/howto/plugins/schema_packages.md

docs/index.md

docs/reference/glossary.md

docs/reference/plugins.md

examples/metainfo/use_metainfo.py

examples/plugins/app.py

examples/plugins/columns.py

examples/plugins/dashboard.py

examples/plugins/menu.py

mkdocs.yml

nomad/config/models/plugins.py

nomad/config/models/ui.py