[2.4] Add reliable xgboost documentation

[YuanTingHsieh]

Description

Add reliable xgboost documentation

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[chesterxgchen]

I felt this is not a complete documentation, simply add the design doc to the user guide is not enough. We really need to add a higher level documentation to explain what's this feature, why it is needed, what's the potential trade offs etc. instead directly jump into implementation and configuration.

Although some of the information is already provided, we have surface up in the user-level doc. The implementation design doc is for those who interested in the details

[chesterxgchen]

Let's improve later. approve for now