Add "copy link" anchors to headings

[iphydf]

Github seems to use this:

<svg aria-hidden="true"
     class="octicon octicon-link"
     height="16"
     version="1.1"
     viewBox="0 0 16 16"
     width="16">
  <path fill-rule="evenodd"
        d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z">
  </path>
</svg>

[cebe]

here is how I made this:

• CSS: https://github.com/yiisoft/yii2-apidoc/blob/master/templates/bootstrap/assets/css/style.css#L170-L176

• HTML:

  <h1>
  Yii PHP Framework Version 2
  <span id="yii-php-framework-version-2"></span>
  <a class="hashlink" href="#yii-php-framework-version-2">¶</a>
  </h1>

See in action here: http://www.yiiframework.com/doc-2.0/guide-README.html