Use `last_modified_at` rather than `last_updated`. So that the
jekyll-sitemap plugin automatically looks for the standard
`last_modified_at` front matter field to generate the `<lastmod>` tag in
your `sitemap.xml`.
It fixes [#2598](https://github.com/alshedivat/al-folio/issues/2598)
Also, it preserves backward compatibility, as the updated metadata logic
prioritizes the `page.last_modified_at`, and defaults to
`page.last_updated`.
This PR is related to #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.
