* Update roxygen2 doc. Co-authored-by: fis <jm.yuan@outlook.com>
* Suggest to use gtest bundled with dmlc * Use dmlc bundled gtest in all CI scripts * Make clang-tidy to use dmlc embedded gtest
* Reorganize contributor's doc * Address comments from @trivialfis * Address @sriramch's comment: include ABI compatibility guarantee * Address @rongou's comment * Postpone ABI compatibility guarantee for now