* [Docs] Make API of docs clearer

* minor fix

* fix md format

* refine metric/evaluator doc

* resolve comments

* resolve comments

* Move the design of hook to design/hook.md

* add relative links in docs

* update docstring of hooks

* refine checkpointhook docs

* refine

* fix comments

* refine

* add logging.md link in hook.md

* resolve comments

* fix typo

* Replace markdownlint with mdformat to avoid installing ruby

* fix circleci

* update docs of metrics and evaluator

* rename file

* resolve comments