feat: add modified_time meta tag

[dimaslanjaka]

add modified date meta tag, useful for SEO

[CookiePLMonster]

This plugin, as well as jekyll-feed, make use of page.last_modified_at. It would make more sense to use this instead of page.updated. This way the SEO tag would match what jekyll-feed outputs:
https://github.com/jekyll/jekyll-feed/blob/43cc5d51e30b59ab8d8ba970e42c8c437e2b233f/lib/jekyll-feed/feed.xml#L78

@ashmaroli otherwise this looks good I think? Pinging you directly as per a comment in #512 (comment) - despite not being the author of this PR, I wish to signal there is interest in this plugin.

[ashmaroli]

I agree with @CookiePLMonster here. It would be better to use the last_modified_at attribute here for party between generated feed and SEO markup.

[CookiePLMonster]

Furthermore, even in this plugin, page.last_modified_at is already used for LD-JSON generation:

jekyll-seo-tag/spec/jekyll_seo_tag/drop_spec.rb

Lines 298 to 315 in f449b1a

	context "date modified" do
	let(:config) { { "timezone" => "America/New_York" } }
	context "with seo.date_modified" do
	let(:page_meta) { { "seo" => { "date_modified" => "2017-01-01" } } }
	it "uses seo.date_modified" do
	expect(subject.date_modified).to eql("2017-01-01T00:00:00-05:00")
	end
	end
	context "with page.last_modified_at" do
	let(:page_meta) { { "last_modified_at" => "2017-01-01" } }
	it "uses page.last_modified_at" do
	expect(subject.date_modified).to eql("2017-01-01T00:00:00-05:00")
	end
	end

Example from my website:

<script type="application/ld+json">
{"@context":"https://schema.org","@type":"BlogPosting","author":{"@type":"Person","name":"Silent"},"dateModified":"2025-04-23T13:30:00+00:00","datePublished":"2025-04-23T13:30:00+00:00"

[CookiePLMonster]

@ashmaroli Looking at

jekyll-seo-tag/lib/jekyll-seo-tag/drop.rb

Lines 114 to 123 in f449b1a

	def date_modified
	@date_modified ||= begin
	date = page_seo["date_modified"] || page["last_modified_at"].to_liquid || page["date"]
	filters.date_to_xmlschema(date) if date
	end
	end
	def date_published
	@date_published ||= filters.date_to_xmlschema(page["date"]) if page["date"]
	end

, the current article:published_time isn't exactly great either and instead it should be seo_tag.published_time - it's used for LD-JSON already, but not for the HTML template. I can prepare an alternative PR tomorrow correcting this + modification date.

[CookiePLMonster]

Additionally, this duplicates #447, albeit using last_modified_at.