[Docs] reorganize the documents in English and update the contents based on v0.5.0 status

[lvhan028]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily receiving feedbacks. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Lots of features are being introduced to v0.5.0. And user documents should be updated accordingly.

Modification

• README.md
• Reorganize the structure of documents listed in docs/en/*
• Upate get_started.md

[tpoisonooo]

CI link check 加一下 en 目录吧。

[tpoisonooo]

• how-to-evaluate-model 和 how-to-measure-performance 实际上在讲同一个命令，所以在 zh_cn 里是一个 profile_model.md
• zh_cn 还有 ncnn 量化文档

[tpoisonooo]

如果想中英文件名+段落完全对齐，推荐个神器 BeyondCompare，两边 setup 一下 dir 可看 diff

[lvhan028]

• how-to-evaluate-model 和 how-to-measure-performance 实际上在讲同一个命令，所以在 zh_cn 里是一个 profile_model.md
• zh_cn 还有 ncnn 量化文档

这两份文档虽然是同一个test.py命令，但是测试metric，和测试速度的arguments不一样。计划暂时保留，之后再更新到一个文档中。
zh_cn 中 ncnn 文档的英文版本计划放在下一个版本中。现在有些来不及