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.

[CookiePLMonster]

Are there any blockers preventing this from moving forward?

[mattr-]

Time, energy, and attention. Jekyll does not have any full time maintainers and we all have other demands on our time that are keeping most things from moving forward at the moment. Your patience is appreciated. ❤️