add last updated date to posts (alshedivat#2341)

This PR is related to alshedivat#2309. I added an optional `last_updated` field.
This new field represents the timestamp for when the post was last
updated. Consequently, the existing `date` metadata should now be
interpreted as the creation date of the post.

The formatting for displaying these dates has been standardized as
follows:

```text
Created: July 11, 2023   |   Last Updated: April 14, 2024
```

For a practical implementation example, please refer to [this
post](https://torydeng.github.io/blog/2023/deploying-server/) on my
website, where I have applied these changes.

---

Any feedback is wellcome.

_layouts/post.liquid

@@ -18,8 +18,9 @@ layout: default
   <header class="post-header">
     <h1 class="post-title">{{ page.title }}</h1>
     <p class="post-meta">
-      {{ page.date | date: '%B %d, %Y' }}
-      {% if page.author %}• {{ page.author }}{% endif %}
+      Created in {{ page.date | date: '%B %d, %Y' }}
+      {% if page.author %}by {{ page.author }}{% endif %}
+      {% if page.last_updated %}, last updated in {{ page.last_updated | date: '%B %d, %Y' }}{% endif %}
       {% if page.meta %}• {{ page.meta }}{% endif %}
     </p>
     <p class="post-tags">

_posts/2024-01-27-vega-lite.md

@@ -2,6 +2,7 @@
 layout: post
 title: a post with vega lite
 date: 2024-01-27 00:20:00
+last_updated: 2024-04-14 04:30:00
 description: this is what included vega lite code could look like
 tags: formatting charts
 categories: sample-posts