* [Refactor] Use mdformat instead of markdownlint to format markdown.
* Update unavailiable api links in tutorials
* Update CONTRIBUTING.md
* Use mdformat==0.7.9 to support Python 3.6
* [Docs] The template provides united Docs and OpenMMLab drop-lists in
banner. Remove them from our `conf.py`.
* Move docs of different languages to the same folder