Improve API document generation

* Create separate directory `doxygen` 
* Split out the main page content from BodyNode.h
* Add DART logo the the main page
* Use default documentation style of `doxygen`

Usage 1 (to generate API document only when there is no generated document):
```
make docs
```
Usage 2 (to enforce to generate API document anyways):
```
make docs_forced
```