Restruct Doc chapters if necessary

[JingL1]

The ADCaaS doc can be re-organized as below if more content added in the future.

1. Introduction: basic concepts of ADCaaS
2. Getting Started: a guide for brand new users
3. Basics: Prerequisite / Installation / Configuration
4. Advanced: advanced details of ADCaaS
5. FAQ: issues based on user feedback
6. Appendix: Glossary & Recipes

Reference: https://github.com/ruanyf/document-style-guide/blob/master/docs/structure.md