Feature/rss feed (#192)

* installed @11ty/eleventy-plugin-rss package and added it to the config

* adding default rss template from the 11ty documentation

* using 'definedWords' as a collection, not 'posts'

* added a link to the feed in the html head, on the homepage so it's co-located with the other means of engagement, and in the feed itself (permalink is now part of metadata too)

* metadata.description is what we want to show as the feed subtitle, and made the author name safe as part of the metadata json; can't use the safe filter otherwise xml blows up

* removed redundant --- from right below the frontmatter, which md turns into a <hr>, which makes xml blow up

* made indentation consistent

* removing subscribe link from the CTA box, adding it to the footer next to Documentation link with bullet delimiter; updated site footer's ul style to show list items as display-block elements

* on definition pages, the footer is constrained to have the max width of the sidebar (160px). set grid-column: 1 / -1 to give it the full width to match the home/toc page

* created a new collection for sorting only defined words via their .date value (should be created date unless otherwise specified)

* RSS feed now shows posts in the order used by new definedWordsChronological collection

* reversed sort order as requested

* re-prettier-ifying eleventy.js

Co-authored-by: Oscar <ovlb@users.noreply.github.com>

assets/css/structures/_footer.scss

@@ -1,3 +1,8 @@
 .site-footer {
+  grid-column: 1 / -1;
   margin-top: 3rem;
+
+  ul > li {
+    display: inline-block;
+  }
 }