docs: add link for QR Code

[alvinhui]

图像看起来会被 Github 缓存，需要重新发布一下，另外加了外链。

[codecov-io]

Codecov Report

Merging #2968 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2968   +/-   ##
=======================================
  Coverage   65.54%   65.54%           
=======================================
  Files          20       20           
  Lines         859      859           
  Branches       47       47           
=======================================
  Hits          563      563           
  Misses        283      283           
  Partials       13       13

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[imsobear]

缓存应该跟语法没关系，是 oss 的缓存吧。我记得有个什么工具可以强刷 cdn 缓存。

[alvinhui]

@imsobear 看了一下 Github REAMDE 里生成的 HTML 里 img src 加载的是 https://camo.githubusercontent.com/95a32bd1234b271f5086ef96f892a11ba517373d/68747470733a2f2f6963652e616c6963646e2e636f6d2f6173736574732f696d616765732f7172636f64652e706e67

好像是 Github 又生成了一次快照？

[alvinhui]

可以参考这篇文章的描述删除 Github 缓存：https://help.github.com/en/github/authenticating-to-github/about-anonymized-image-urls#removing-an-image-from-camos-cache