Refactoring ORT Trainers documentation (#376)
* New design for the doc of ORT Trainers
* Add installation guide
* Expand installation details
* Add other prerequistite
* Add other prerequistite
* Add doc for training arguments
* Fix style of args doc
* Fix example url
* Fix typo
* Fix indention
* Complete the args for ORTTrainer
* Test link to internal libraries
* Complete docstring for functions in ORTTrainer
* Complete docstring for functions in ORTSeq2SeqTrainer
* Modify according to comments
* Fix typos
