Add hierarchy documents including introduction, database
management,defining node, service node setup, test service
node and appendix.
TODO: resolve the fake_todo link reference in the following patch
- categorize into topics without "xCAT" in the heading
since the topics are all related to xCAT
- Alphabetize the topics to make it easier to find
- create sub directories for the topics to contain
the documentation
- broke down some of the docs to multiple files to help
ease of readability
