Added post citation (alshedivat#2377)

Basically adds suggestions of how to cite a post, as suggested in alshedivat#2374.


![image](https://github.com/alshedivat/al-folio/assets/31376482/756eb88a-35c9-435a-b79e-70d11aa489cb)

---------

Signed-off-by: George Araujo <george.gcac@gmail.com>

_includes/citation.liquid

@@ -0,0 +1,26 @@
+<br>
+<hr>
+<br>
+
+If you found this useful, please cite this as:
+
+{% capture citation_quote -%}
+> {{ site.last_name }}, {{ site.first_name }}{% if site.middle_name %} {{ site.middle_name }}{% endif %} ({{ page.date | date: "%b %Y" }}). {{ page.title }}. {% if site.title != 'blank' %}{{ site.title }}. {% endif %}{{ site.url }}.
+{%- endcapture %}
+{{ citation_quote | markdownify }}
+
+<p>or as a BibTeX entry:</p>
+
+{% capture citation_code -%}
+```bibtex
+@article{ {{- site.last_name | downcase }}{{ page.date | date: "%Y" }}{{ page.title | slugify }},
+  title   = { {{- page.title -}} },
+  author  = { {{- site.last_name }}, {{ site.first_name }}{% if site.middle_name %} {{ site.middle_name }}{% endif -%}},
+  {%- if site.title != 'blank' %}journal = { {{- site.title -}} },{% endif %}
+  year    = { {{- page.date | date: "%Y" -}} },
+  month   = { {{- page.date | date: "%b" -}} },
+  url     = { {{- site.url }}{{ page.url -}} }
+}
+```
+{%- endcapture %}
+{{ citation_code | markdownify }}

_layouts/post.liquid

@@ -69,6 +69,10 @@ layout: default
     </div>
   </article>
 
+  {% if page.citation %}
+    {% include citation.liquid %}
+  {% endif %}
+
   {% if page.related_publications %}
     <h2>References</h2>
     <div class="publications">

_posts/2024-04-28-post-citation.md

@@ -0,0 +1,11 @@
+---
+layout: post
+title: a post that can be cited
+date: 2024-04-28 15:06:00
+description: this is what a post that can be cited looks like
+tags: formatting citation
+categories: sample-posts
+citation: true
+---
+
+This is an example post that can be cited. The content of the post ends here, while the citation information is automatically provided below. The only thing needed is for you to set the `citation` key in the front matter to `true`.