Added documentation for the new plugin mechanism.

dependencies/normalizers/simulation/band_structure

dependencies/normalizers/simulation/dos

dependencies/normalizers/simulation/soap

dependencies/normalizers/simulation/spectra

dependencies/normalizers/simulation/system

dependencies/normalizers/simulation/workflow

docs/examples/computational_data/parser_plugins.md

docs/explanation/basics.md

docs/explanation/data.md

docs/explanation/processing.md

docs/howto/customization/basics.md

docs/howto/customization/normalizers.md

docs/howto/customization/plugins.md

docs/howto/develop/contrib.md

docs/howto/oasis/apps.md

docs/howto/oasis/customize.md

docs/howto/oasis/plugins_install.md

docs/howto/overview.md

docs/howto/plugins/apps.md

docs/howto/plugins/normalizers.md