* Update the contributors list.
* Update the version number.
* Sync zh and zh-hant versions.
* Move epub builder to the utils branch.
* Optimize all figure files using pngquant.
* Update copyright.
* Update Acknowledgements.
* Update buttons in README.
* Sync zh and zh-hant versions.
* Remove the polyfill.io link from mkdocs.yml
* Update contributors' info for code reviewers and en/zh-hant versions reviewers.
* Fix graph.md
* Update avatars for English version reviewers.
* Sync zh and zh-hant versions.
* Fix two_sum_brute_force.png
* Sync zh and zh-hant versions.
Optimize structrue of index.html.
* Format index.html