Adds atom template with logic for multiple RSS feeds (#327)

### Description

This pulls in source for the pre-built atom feed from zola, adds logic
to filter by "section", overrides the pre-built atom feed, and enables
an event-only feed.

In the end, the main atom feed (`/atom.xml`) is just blog posts. There
is now an additional atom feed for events section `/events/atom.xml`)
and for any future sections that enable the feed generaiton.


### Issues Resolved

#322

### Check List
- [x] Commits are signed per the DCO using `--signoff`

By submitting this pull request, I confirm that my contribution is made
under the terms of the BSD-3-Clause License.

---------

Signed-off-by: Kyle J. Davis <[email protected]>
Signed-off-by: Madelyn Olson <[email protected]>
Co-authored-by: Madelyn Olson <[email protected]>

Author: stockholmux

content/events/_index.md

@@ -2,4 +2,5 @@
 title = "Events"
 template = "events.html"
 page_template = "event-page.html"
+generate_feeds = true
 +++

templates/atom.xml

@@ -0,0 +1,57 @@
+{# built from https://github.com/getzola/zola/blob/master/components/templates/src/builtins/rss.xml / MIT #}<?xml version="1.0" encoding="UTF-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom" xml:lang="{{ lang }}">
+    <title>{{ config.title }}
+    {%- if term %} - {{ term.name }}
+    {%- elif section.title %} - {{ section.title }}
+    {%- endif -%}
+    </title>
+    {%- if config.description %}
+    <subtitle>{{ config.description }}</subtitle>
+    {%- endif %}
+    <link rel="self" type="application/atom+xml" href="{{ feed_url | safe }}"/>
+    <link rel="alternate" type="text/html" href="
+      {%- if section -%}
+        {{ section.permalink | escape_xml | safe }}{%- set section_id = section.components[0] -%}
+      {%- else -%}
+        {{ config.base_url | escape_xml | safe }}
+      {%- endif -%}
+    "/>
+
+    <generator uri="https://www.getzola.org/">Zola</generator>
+    {% if last_updated is defined %}<updated>{{ last_updated | date(format="%+") }}</updated>{% endif %}
+    <id>{{ feed_url | safe }}</id>
+    {%- for page in pages %}
+    
+    {%- if (page.components[0] == "blog") or (section_id and section_id == page.components[0])  -%}
+    <entry xml:lang="{{ page.lang }}">
+        <title>{{ page.title }}</title>
+        <published>{{ page.date | date(format="%+") }}</published>
+        <updated>{{ page.updated | default(value=page.date) | date(format="%+") }}</updated>
+        {% for author in page.authors %}
+        <author>
+          <name>
+            {{ author }}
+          </name>
+        </author>
+        {% else %}
+        <author>
+          <name>
+            {%- if config.author -%}
+              {{ config.author }}
+            {%- else -%}
+              Unknown
+            {%- endif -%}
+          </name>
+        </author>
+        {% endfor %}
+        <link rel="alternate" type="text/html" href="{{ page.permalink | safe }}"/>
+        <id>{{ page.permalink | safe }}</id>
+        {% if page.summary %}
+        <summary type="html">{{ page.summary }}</summary>
+        {% else %}
+        <content type="html" xml:base="{{ page.permalink | escape_xml | safe }}">{{ page.content }}</content>
+        {% endif %}
+    </entry>
+    {%- endif -%}
+    {%- endfor %}
+</feed>