RSS Autodiscovery for rss/json feed (#714)

Signed-off-by: Daniel Gray <dng@disroot.org>

feed.json

@@ -0,0 +1,25 @@
+---
+layout: null
+---
+{
+    "version": "https://jsonfeed.org/version/1",
+    "title": "{{ site.title | xml_escape }}",
+    "description": {{ site.description | jsonify }},
+    "home_page_url": "{{ "/" | absolute_url }}",
+    "feed_url": "{{ "/feed.json" | absolute_url }}",
+    "user_comment": "This feed allows you to read the posts from this site in any feed reader that supports the JSON Feed format.",
+    "items": [{% for post in site.posts %}
+        {
+            "id": "{{ post.url | absolute_url }}",
+            "url": "{{ post.url | absolute_url }}",
+            "title": {{ post.title | jsonify }},
+            "content_html": {{ post.content | jsonify }},
+            "date_published": "{{ post.date | date_to_xmlschema }}",
+            "authors": [{% for author in post.author %}
+                {
+                    "name": "{{ author }}"
+                }{% unless forloop.last %}, {% endunless %}{% endfor %}
+            ]
+        }{% unless forloop.last %},{% endunless %}{% endfor %}
+    ]
+}