Add a article:modified_time property for articles

[CookiePLMonster]

This PR adds support for article:modified_time meta property, utilizing the same seo_tag drop values as JSON-LD does.

Two other PRs adding this feature (#447 and #505) are also open, but my attempt has the following advantages:

1. seo_tag drop values are used for complete consistency with the JSON-LD output. This ensures compatibility with bothpage.last_modified_at and seo.date_modified front matter attributes. I also extended this to the published date instead of using page.date directly.
2. Updated the Advanced Usage docs to mention that seo.date_modified also influences this new meta property.

Verified working using my own website - on a test blog post, I can see a new meta property added:

<meta property="og:type" content="article" />
<meta property="article:published_time" content="2025-04-23T15:30:00+02:00" />
<meta property="article:modified_time" content="2025-04-23T15:30:00+02:00" />

The behaviour of adding the modification time even if it equals the publishing time matches jekyll-feed.

[CookiePLMonster]

@ashmaroli I created this PR in response to our discussion in #505 - whichever PR ends up being merged, the other one (as well as #447) can be closed.