Add zh-CN translation to intro

[pickfire]

Fix #292

I added it to intro rather than readme because it's the first page seen in the website, having it in the first place will make it more discoverable.

[simonsan]

I would probably rather do it this way, still not sure though.

• Create a translations.md and add it to SUMMARY.md as a subcategory under Introduction.
• Then link to the translation from there

./SUMMARY.md

- [Introduction](./intro.md)
  - [Translations](./translations.md)

./translations.md

# Translations

- [简体中文翻译](https://fomalhauthmj.github.io/patterns/)

If you want to add a translation, please open an issue in the [main repository](https://github.com/rust-unofficial/patterns).

[Fomalhauthmj]

@pickfire, I think @simonsan's approach is better, can you push a new commit?

[pickfire]

@simonsan Thanks, I have updated it as what you said. I didn't know summary is the main entry file.