added subsection to docs index

docs/guide/index.md

-* [Overview](overview.md)
-* [Installation](installation.md)
-* [Bootstrap with Yii](bootstrap.md)
-* [MVC Overview](mvc.md)
-* [Controller](controller.md)
-* [Model](model.md)
-* [View](view.md)
-* [Application](application.md)
-* [Form](form.md)
-* [Data Validation](validation.md)
-* [Database Access Objects](dao.md)
-* [Query Builder](query-builder.md)
-* [ActiveRecord](active-record.md)
-* [Database Migration](migration.md)
-* [Caching](caching.md)
-* [Internationalization](i18n.md)
-* [Extending Yii](extension.md)
-* [Authentication](authentication.md)
-* [Authorization](authorization.md)
-* [Logging](logging.md)
-* [URL Management](url.md)
-* [Theming](theming.md)
-* [Error Handling](error.md)
-* [Template](template.md)
-* [Console Application](console.md)
-* [Security](security.md)
-* [Performance Tuning](performance.md)
-* [Testing](testing.md)
-* [Automatic Code Generation](gii.md)
-* [Upgrading from 1.1 to 2.0](upgrade-from-v1.md)
+Introduction
+============
+
+- [Overview](overview.md)
+
+Getting started
+===============
+
+- [Installation](installation.md)
+- [Bootstrap with Yii](bootstrap.md)
+
+Base concepts
+=============
+
+- [MVC Overview](mvc.md)
+- [Controller](controller.md)
+- [Model](model.md)
+- [View](view.md)
+- [Application](application.md)
+- [Form](form.md)
+- [Model validation reference](validation.md)
+
+Database
+========
+
+- [Database Access Objects](dao.md)
+- [Query Builder](query-builder.md)
+- [ActiveRecord](active-record.md)
+- [Database Migration](migration.md)
+
+More
+====
+
+- [Caching](caching.md)
+- [Internationalization](i18n.md)
+- [Extending Yii](extension.md)
+- [Authentication](authentication.md)
+- [Authorization](authorization.md)
+- [Logging](logging.md)
+- [URL Management](url.md)
+- [Theming](theming.md)
+- [Error Handling](error.md)
+- [Template](template.md)
+- [Console Application](console.md)
+- [Security](security.md)
+- [Performance Tuning](performance.md)
+- [Testing](testing.md)
+- [Automatic Code Generation](gii.md)
+- [Upgrading from 1.1 to 2.0](upgrade-from-v1.md)