a11y(blocks): render collection/markdown section titles as <h2>

[Vidminas]

The collection and markdown blocks rendered their title as a styled <div class="text-3xl">, so section titles ("Recent Publications", "Recent News", …) never appeared in the document heading outline and screen-reader heading navigation skipped them (WCAG 1.3.1).

🚀 What type of change is this?

• 🐛 Bug fix (A non-breaking change that fixes an issue)
• ✨ New feature (A non-breaking change that adds functionality)
• 💅 Style change (A change that only affects formatting, visuals, or styling)
• 📚 Documentation update (Changes to documentation only)
• 🧹 Refactor or chore (A code change that neither fixes a bug nor adds a feature)
• 💥 Breaking change (A fix or feature that would cause existing functionality to not work as expected)

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🎯 What is the purpose of this change?

The collection and markdown blocks render their section title as a styled
<div class="text-3xl">. So headings like "Recent Publications", "Recent News", etc. don't
appear in the document heading outline, and screen-reader heading navigation (a primary way AT
users skim a page) skips them entirely (WCAG 1.3.1).

Fix. Render the title as <h2>, keeping the existing visual classes. This matches the
portfolio block, which already uses <h2> for its title — so it also makes the three blocks
consistent.

Verified on the academic-cv starter: the homepage now exposes five <h2> section headings
("📚 My Research", "Featured Publications", "Recent Publications", "Recent & Upcoming Talks",
"Recent News") with no remaining title <div>s.

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📸 Screenshots or Screencast (if applicable)

Although it's primarily a semantic fix, because the title becomes an <h2> it now inherits the theme's heading font (and heading line-height/tracking) instead of the body font — the same font the portfolio block's title already uses. On the academic-cv starter that's Lora (heading) vs Source Serif 4 (body); see the screenshots below. The text-3xl font-bold size/weight classes are unchanged, so only the typeface shifts.

The same "Recent Publications" title rendered both ways, with the real webfonts loaded:

The effect is theme-independent; both themes are shown for completeness.

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ℹ️ Documentation Check

• No, this change does not require a documentation update.
• Yes, I have updated the documentation accordingly (or will in a follow-up PR).

No config or API changes. Worth noting: the markdown block's own README already claims
"Accessibility First: Semantic HTML structure with proper heading hierarchy" — which was
inaccurate while the title rendered as a <div>. This change makes that existing claim true, so no
doc edit is needed (it's the code catching up to the docs).

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