* Add "reference" for EN version. Bug fixes.
* Unify the figure reference as "the figure below" and "the figure above".
Bug fixes.
* Format the EN markdown files.
* Replace "" with <u></u> for EN version and bug fixes
* Fix biary_tree_dfs.png
* Fix biary_tree_dfs.png
* Fix zh-hant/biary_tree_dfs.png
* Fix heap_sort_step1.png
* Sync zh and zh-hant versions.
* Bug fixes
* Fix EN figures
* Bug fixes
* Fix the figure labels for EN version
* Capitalize all the headers, list headers and figure captions
* Fix the term "LRU"
* Fix the names of source code link in avl_tree.md
* Capitalize only first letter for nav trees in mkdocs.yml
* Update code comments
* Update linked_list.md
* Update linked_list.md
* Update pythontutor block with the latest code
* Move docs-en to en/docs
* Move mkdocs.yml and README to en folder
* Fix en/mkdocs.yml
* Update the landing page
* Fix the glossary
* Reduce the font size of the code block tabs
* Add Kotlin blocks to en/docs
* Fix the code link in en/.../deque.md
* Fix the EN README link
Yudong Jin