Adopt skainet-docs-ui v1.1.1 (#53)

[michalharakal]

Closes #53.

Visual alignment with mainline SKaiNET. Bumps `docs/antora-playbook.yml`'s
`ui.bundle.url` to the new shared SKaiNET-branded Antora UI bundle from
`SKaiNET-developers/skainet-docs-ui`,
pinned to `v1.1.1`. Mainline SKaiNET pins the same bundle in
SKaiNET#502.

Two-line scope

```diff
ui:
bundle:

• url: https://gitlab.com/antora/antora-ui-default/-/jobs/artifacts/HEAD/raw/build/ui-bundle.zip?job=bundle-stable

• url: https://github.com/SKaiNET-developers/skainet-docs-ui/releases/download/v1.1.1/ui-bundle.zip
  snapshot: true
  ```

What you'll see in the rebuilt site

• Brand color: SKaiNET red `#dc2626` instead of upstream Antora blue
• Typography: Orbitron headings, Inter body, Fira Code monospace (was Roboto everywhere)
• Dark mode toggle in the navbar (🌓 button), persists via `localStorage`, defaults from `prefers-color-scheme`, FOUC-safe via inline head bootstrap
• Code blocks: GitHub-light `#f6f8fa` in light mode, GitHub-dark `#0d1117` in dark mode, 8px rounded corners, 1px border
• Scrollbar: 6px thin
• Focus glow: primary-red on `:focus-visible` for links / buttons / nav items
• Footer: replaces upstream "Antora default UI / MPL-2.0" boilerplate with SKaiNET attribution + cross-repo links to mainline + skainet-docs-ui
• Mobile: responsive hamburger nav (preserved from upstream)
• `.card-grid` CSS: available but unused on this site (the existing Key Features / Supported Model Families landing content is preserved)

Compatibility notes

• The new UI bundle does NOT touch asciidoctor extensions. The existing `asciidoctor-kroki` extension and `kroki-fetch-diagram: true` config in this playbook continue to work unchanged — mermaid rendering follows the same pipeline as before.
• The bundle is content-neutral: nav entries, page content, and per-page metadata still come from this repo's modules and playbook. Only visual chrome (CSS, JS, partials) comes from the bundle.

Out of scope

• Rewriting the landing page with a Diátaxis card-grid. The transformers landing has substantial real content (Key Features, Supported Model Families table) and shouldn't be replaced wholesale. A future PR can add a card-grid SECTION above or below the existing content if useful.
• Self-hosted fonts. v1.1.1 still loads Orbitron / Inter / Fira Code from Google Fonts; can move to bundled `@font-face` files when skainet-docs-ui v1.2 ships.
• Search wiring (`SITE_SEARCH_PROVIDER` env var still unset; the UI ships an input box already, just needs a search backend).

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