Add pandoc css styling for `task-list`

[John Benninghoff]

From jgm/pandoc#8151, pandoc uses CSS to render GitHub markdown tasklists.

pkgdown currently renders tasklists as bulleted lists with no space between the checkbox and the text. Importing the pandoc task-list styling from https://github.com/jgm/pandoc/blob/main/data/templates/styles.html to pkgdown/extra.css corrects this issue (contents below). Can the CSS below be added to the pkdown default CSS?

/* pandoc task-list styling from https://github.com/jgm/pandoc/blob/main/data/templates/styles.html */
/* The extra [class] is a hack that increases specificity enough to
   override a similar rule in reveal.js */
ul.task-list[class]{list-style: none;}
ul.task-list li input[type="checkbox"] {
  font-size: inherit;
  width: 0.8em;
  margin: 0 0.8em 0.2em -1.6em;
  vertical-align: middle;
}

[Jay Hesselberth]

This doesn't seem generally useful in a static site, and is easily accommodated using extra.css, so we'll not likely add this feature to pkgdown. Closing for now.