Add rss feed to head for each page

Currently the RSS feed is only listed in the head of https://monthly-reports.archlinux.page/; putting the URL of any report into a feed reader hence fails to find any feed.

This adds a partial as documented in the hugo-book theme to inject the RSS URL into the head of report pages; to avoid a duplicate feed link it first checks whether the current page is not already the homepage (which already has a feed link). Then it looks up the home page and extracts its RSS link, to avoid building the link manually.