[ci skip] 📝 Modified mkdocs.yml to fit new mkdocs format

mkdocs.yml

 site_name: Sequelize | The Node.js / io.js ORM for PostgreSQL, MySQL, SQLite and MSSQL
 site_description: Sequelize is an ORM for Node.js and io.js. It supports the dialects PostgreSQL, MySQL, MariaDB, SQLite and MSSQL.
-theme: ""
-theme_dir: docs/readthedocs_theme
+# theme_dir: docs/readthedocs_theme
 repo_url: https://github.com/sequelize/sequelize
 site_favicon: favicon.ico
 site_url: http://docs.sequelizejs.com
@@ -11,32 +10,32 @@ extra_css:
 extra_javascript:
 - //cdnjs.cloudflare.com/ajax/libs/highlight.js/8.4/highlight.min.js
 pages:
-- ['index.md', 'Home']
-
-- ['docs/getting-started.md', 'Documentation', 'Getting Started']
-- ['docs/schema.md', 'Documentation', 'Working with table schemas']
-#- ['docs/usage.md', 'Documentation', 'Usage']
-- ['docs/models-definition.md', 'Documentation', 'Models - Definition']
-- ['docs/models-usage.md', 'Documentation', 'Models - Usage']
-- ['docs/querying.md', 'Documentation', 'Querying']
-- ['docs/scopes.md', 'Documentation', 'Scopes']
-- ['docs/instances.md', 'Documentation', 'Instances']
-- ['docs/associations.md', 'Documentation', 'Relations/Associations']
-- ['docs/hooks.md', 'Documentation', 'Hooks']
-- ['docs/transactions.md', 'Documentation', 'Transactions']
-- ['docs/legacy.md', 'Documentation', 'Working with legacy tables']
-- ['docs/raw-queries.md', 'Documentation', 'Raw queries']
-- ['docs/migrations.md', 'Documentation', 'Migrations']
-
-- ['api/sequelize.md', 'API', 'Sequelize']
-- ['api/model.md', 'API', 'Model']
-- ['api/instance.md', 'API', 'Instance']
-- ['api/associations.md', 'API', 'Associations']
-- ['api/hooks.md', 'API', 'Hooks']
-- ['api/transaction.md', 'API', 'Transaction']
-- ['api/datatypes.md', 'API', 'Datatypes']
-- ['api/deferrable.md', 'API', 'Deferrable']
-- ['api/errors.md', 'API', 'Errors']
-
-- ['changelog.md', 'Misc', 'Changelog']
-- ['imprint.md', 'Misc', 'Imprint']
+- Home: 'index.md'
+- Documentation:
+  - 'Getting Started': 'docs/getting-started.md'
+  - 'Working with table schemas': 'docs/schema.md'
+  - Models:
+    - 'Definition': 'docs/models-definition.md'
+    - 'Usage': 'docs/models-usage.md'
+  - 'Querying': 'docs/querying.md'
+  - 'Scopes': 'docs/scopes.md'
+  - 'Instances': 'docs/instances.md'
+  - 'Relations / Associations': 'docs/associations.md'
+  - 'Hooks': 'docs/hooks.md'
+  - 'Transactions': 'docs/transactions.md'
+  - 'Working with legacy tables': 'docs/legacy.md'
+  - 'Raw queries': 'docs/raw-queries.md'
+  - 'Migrations': 'docs/migrations.md'
+- API:
+  - 'Sequelize': 'api/sequelize.md'
+  - 'Model': 'api/model.md'
+  - 'Instance': 'api/instance.md'
+  - 'Associations': 'api/associations.md'
+  - 'Hooks': 'api/hooks.md'
+  - 'Transaction': 'api/transaction.md'
+  - 'Datatypes': 'api/datatypes.md'
+  - 'Deferrable': 'api/deferrable.md'
+  - 'Errors': 'api/errors.md'
+- Misc:
+  - 'Changelog': 'changelog.md'
+  - 'Imprint': 'imprint.md'