Added new plugin entry point type for example uploads.

docs/howto/plugins/example_uploads.md

docs/howto/plugins/plugins.md

docs/reference/plugins.md

examples/data/cow_tutorial/nomad-countries/pyproject.toml

gui/src/components/uploads/ExampleUploadButton.js

gui/src/config.js

mkdocs.yml

nomad/app/v1/routers/uploads.py

nomad/cli/dev.py

nomad/common.py

nomad/config/models/config.py

nomad/config/models/plugins.py

nomad/config/models/ui.py

nomad/files.py

tests/app/v1/routers/uploads/test_basic_uploads.py

tests/config/models/test_common.py

tests/config/models/test_plugins.py

tests/data/proc/example_upload/a.txt

tests/data/proc/example_upload/a/b.txt

tests/test_files.py