docs(showcase): add Layered Architecture callout under hero

[DemchaAV]

Summary

Right now the GitHub Pages showcase is a gallery of generated PDFs — useful, but it doesn't surface the new v2 layered template architecture or its documentation. Visitors arrive, browse the 14 v1 CV presets, and have no obvious path to "I want to build my own template."

This PR drops a small callout block between the hero and the value-prop strip that points at the v2 layered docs.

What changed

docs/index.html

• New <section class="layered-callout"> between hero and value-prop strip:
  • NEW pill badge (tinted gradient, matches existing accent palette)
  • Title: Building your own templates?
  • One-sentence intro describing the 5-folder pattern (data / theme / components / widgets / presets)
  • 3 buttons:
    • Quickstart (5 min) → docs/templates/v2-layered/README.md
    • Author a preset → docs/templates/v2-layered/authoring-presets.md
    • Add a template family → docs/templates/v2-layered/contributor-guide.md

docs/styles.css

• New .layered-callout / .layered-card / .layered-badge / .layered-actions rules.
• Dark-theme tokens (slate-50 / slate-200 / slate-300) consistent with the rest of the page. First iteration accidentally used dark text on the dark background — fixed.

Screenshot

Drag docs/private/showcase-callout-fullpage.png into this description box to embed.

The callout sits directly under the hero, above the 30 lines → one invoice / Atomic pagination / 14 CV presets / Tested at every layer grid. NEW badge, three buttons with distinct visual weights (primary cyan → secondary outline → ghost gold).

What this is NOT

A placeholder discoverability fix, not a showcase rework. A proper Level 1 overhaul (hero with persona buttons, v2 preset cards with doc-links, 5-layer SVG diagram, v1 collapsed by default) is deferred until v2 stabilises across 3+ ported presets or the first non-CV template family ships. Full plan is recorded in docs/private/cv-v2-roadmap.md for when that moment arrives.

Scope

• No engine changes.
• No source / test changes.
• HTML + CSS only — affects the GitHub Pages site, nothing else.

Test plan

• Verify the callout renders correctly on the deployed GitHub Pages site after merge.
• Click each of the 3 buttons — all 3 should land on the correct doc page.
• Confirm text contrast is readable on the dark theme (already verified locally).