refactor: update blog post date handling and schema

- Replaced `pubDate` with `datePublished` in blog post components for consistency.
- Updated sorting logic in blog sections to use `datePublished`.
- Enhanced blog post schema to include `dateModified` for better structured data representation.
- Adjusted various blog markdown files to reflect the new date fields.

src/content/blog/example-content.md

@@ -1,8 +1,9 @@
 ---
 title: "Example Content"
-author: "Example User"
-pubDate: "2018-01-01"
 description: "Howdy! This is an example blog post that shows several types of HTML content supported in this theme."
+datePublished: "2018-01-01"
+dateModified: "2018-01-01"
+author: "Example User"
 lang: "en"
 draft: true
 ---