refactor: re-organize all docs

docs/dataset.md

docs/img/create_instance.png

docs/img/handle_databases.png

docs/img/handle_databases_configuration.png

docs/img/import_instance.png

docs/img/instance_design.png

docs/img/instance_edition.png

docs/img/instance_setting.png

docs/instance.md

docs/web_admin.md

mkdocs.yml