blog: Make headings clickable, for deep-linking

[ee7]

It is currently possible to add e.g. # to the end of the URL, like: https://nim-lang.org/blog/2020/10/16/version-140-released.html#changelog

But I'd like for it to be more obvious.

One common implementation is a little link icon on mouseover, like in: https://github.com/nim-lang/website/blob/master/jekyll/_posts/2020-10-16-version-140-released.md#changelog

That makes it more obvious that it's not a link to another page.