add epub generator (#1831)

* add epub generator

* improve parser, keep images

* check epub after generate

* fix render math content error in block

* render \dots as ...

* render \lfoor and \rfloor

* use monospaced font to render code block

* render code block with syntax highlight

* adjust title render

* fix render LaTeX

* fix '!!! abstract' render

* render code block in flow

* include whole class when not specifiy function

* command line to build other language

* update README

* fix process python code example

* support build en, ja and zh-hant

* add '--all' option to build all version

* use branch docs to build epub

* fix title and toc render

* build epub file with name like 'hello-algo_{zh}_{cpp}.epub

* fix render LaTeX

* optimize style

* use math font

* fix extract code block

* add border for code block

* fix python code style

* fix page break

* try git pull first when build epub

* ajust title level of chapter section

* Update epub styles

* Update epub styles

* Update convers and fonts.

* Convert code comments and README into English.

* Update the output dir.

* Add code reviewers on the cover.

* Support multi language for the reviewer names.

* Update .gitignore

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Co-authored-by: krahets <krahets@163.com>