fix: /feed.xml returns unstyled raw XML in the browser

[nkatha23]

Description

Visiting /feed.xml shows a wall of unstyled raw XML, poor UX for anyone who navigates there directly.

Steps to Reproduce

1. Visit /feed.xml on the live site

Proposed Fix

Add an XSLT stylesheet (feed.xslt.xml) and reference it from the feed via an <?xml-stylesheet?> processing instruction by overriding the default feed templates for both Jekyll (local) and Zola (production).