add tutorial on users, roles and privileges management

docs/api.md

docs/glossary.md

docs/images/role_model.svg

docs/images/user_priv.svg

docs/images/user_roles.svg

docs/index.md

docs/sql/ebnf/ACL.svg

docs/sql/ebnf/Column.svg

docs/sql/ebnf/CreateRole.svg

docs/sql/ebnf/CreateUser.svg

docs/sql/ebnf/DropRole.svg

docs/sql/ebnf/DropUser.svg

docs/sql/ebnf/STATEMENT.svg

docs/sql/queries.md

docs/tutorial_data.md

docs/tutorial_first_steps.md

docs/tutorial_users.md

mkdocs.yml