Document the differences between highlight shortcode and code fence attributes

[kaushalmodi]

Hello,

Recently while trying to use the anchorlinenos attribute in code fences, I learnt that that attribute and related lineanchor are available only for the highlight shortcode and not code fences.

In an upcoming PR gohugoio/hugo#9390, the hl_style attribute for code fences will be equivalent to the style attribute of the highlight shortcode.

So I am proposing that a table be added to the highlighting section in the documentation that lists and compares the attributes/switches available for code fences vs the highlight shortcode.