Posts in Hugo theme should have a link to index.xml RSS feed

I think RSS feeds in hugo websites are currently undiscoverable, which is probably why there are basically no application feeds on Planet. The website maintainer would have to know that 1) the file is created by default, and 2) the feed file is called index.xml, and this info is easily missable.

I think it would make sense to have RSS feeds be shown at the bottom of the posts list, so in layouts/_default/list, making things easier for the website maintainer and for users. If index.xml is under the post section, implementing this should be as trivial as adding something like

<p>Follow {{ .Site.Title }} news with our <a href="index.xml">RSS Feed</a></p>

to https://invent.kde.org/websites/aether-sass/-/blob/hugo/layouts/_default/list.html#L37, maybe include the RSS icon too. This works fine for most websites that use the default, like Kate.

I was about to push and make an MR when I tested and noticed that plasma-mobile.org does something different: it puts index.xml in the root folder ("- RSS" was put under Home instead of Section here). Given that, I thought maybe there's a way for Hugo to find the index.xml and put it in the <a href="path/to/index.xml">, but I didn't find a way to do this.