feat(a11y): adopt mod-blocks v2.4.0 so blocks carry heading levels

[markdumay]

Closes the loop on #1519 phase 2. #2116 taught the section title partial to render a heading, gethinode/mod-blocks#193 made the blocks ask for one, and #2122 moved the argument definitions into mod-utils. Until this bump none of it is visible: the example site still pinned mod-blocks v2.3.6, so nothing passed a level and every block title stayed a div.

What changes

Bumps the example site to mod-blocks v2.4.0 (and mod-utils to v6.9.0, pulled along with it).

Seven pages that rendered no heading at all — not a missing h1, nothing — now open with a real one:

page	now
/en/	<h1 id="welcome-to-hinode">
/nl/	<h1 id="welkom-bij-hinode">
/fr/	<h1 id="bienvenue-sur-hinode">
/en/team/	<h1 id="meet-our-team">
/fr/team/	<h1 id="rencontrez-notre-équipe">
/en/docs/	<h1 id="docs">
/en/docs/blocks/, /en/docs/components/	<h1 id="blocks">, <h1 id="components">

Adds blocks-demo, the fixture deferred from the mod-blocks review. It is the only page in the example site carrying both content_blocks and page content, which makes it the only cover for single.html's level-2 branch — the one path the heading work had no fixture for. Its outline is exactly the documented shape:

h2  #a-block-on-a-page-with-content
h2  #what-the-blocks-look-like
h1  #blocks-demo
h2  #regular-page-content

Blocks render above the page header, so their titles are subsections and the page title below them stays the only h1. That ordering is the tradeoff recorded in single.html; this page makes it visible rather than theoretical.

It follows the existing *-demo convention (form-demo, id-demo, lightbox-demo, table-demo) and, like those, is not linked from any menu.

Verification

• 112 pages, exactly one h1 each, no page skips a level. Previously seven had none.
• hugo -s exampleSite builds with no errors and no new warnings.
• test:templates exits 0; markdownlint clean on the new page.

Still open

• mod-blocks: lead scan loses the page's h1 when every titled block sets hide_empty and the first is empty #2121 — the lead scan can still lose the h1 when every titled block sets hide_empty and the first is empty.
• use-title cannot be deprecated yet: mod-blocks still passes it from assets/hero.html and assets/contact.html.
• Consuming sites that use the CloudCannon editor will now see a heading_level field on every block, and can hide it the way gethinode.com already hides bg-class, section-class and overlay-mode.

🤖 Generated with Claude Code

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[markdumay]

🎉 This PR is included in version 3.20.0 🎉

The release is available on GitHub release

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