Indicate whether a link is external

Related: #1605

React docs have lots of links to external articles, videos or even Stack Overflow questions. For example, the "Hello World" page has this:

Note This guide occasionally uses some of the newer JavaScript syntax in the examples. If you haven’t worked with JavaScript in the last few years, these three points should get you most of the way.

While this link is helpful to English speakers, it can easily disappoint readers of translated versions because many of them do not understand English well.

This can be especially unfriendly when they cannot reasonably guess whether the linked material is in English or not from its text or the context ("these points", "here", "said", "ES6 class", "weird bug" etc). Unfortunately, the current docs are full of instances like this.

In the long run, we can reduce the number of such confusing links, for example:

Note This guide occasionally uses some of the newer JavaScript syntax in the examples. If you haven’t worked with JavaScript in the last few years, the following article should get you most of the way.

• Modern JavaScript in React Documentation

But for the time being, I suggest indicating external links using an icon (e.g., 🌎) or a different color. It can signal the linked article is likely to be written in English. Maybe links to an MDN article can have a different icon because it usually has a translated version.

首先检查 React 文档网站中各处链接的呈现方式，并阅读相关 issue #1605 以了解背景。盘点外部链接，包括 MDN 链接，并将以下情况定义为工作完成：读者无需依赖链接文本即可区分外部材料和内部链接。