Adopt skainet-docs-ui v1.1.1 (SKaiNET-branded Antora UI)

[michalharakal]

Visual alignment with mainline SKaiNET. Bump `ui.bundle.url` in
`docs/antora-playbook.yml` to the new shared SKaiNET-branded Antora
UI bundle from
`SKaiNET-developers/skainet-docs-ui`.

Mainline SKaiNET adopted the same bundle in
SKaiNET#502.
This issue is the matching change for the transformers docs site.

What changes for the docs site

The bundle ships a complete visual rebrand on top of the upstream
`antora/antora-ui-default` bundle the playbook currently points at:

• SKaiNET red palette (`#dc2626`) replaces upstream Antora blue
• Orbitron (display) / Inter (body) / Fira Code (mono) replace Roboto
• Light + dark theme toggle in the navbar (FOUC-safe via inline head
  bootstrap, persisted via `localStorage`, respects
  `prefers-color-scheme` on first visit)
• 6px thin custom scrollbar
• Rounded code blocks with GitHub-light / GitHub-dark backgrounds
  (`#f6f8fa` / `#0d1117`)
• Primary-red focus glow on interactive elements
• New SKaiNET footer with cross-repo links to mainline +
  skainet-docs-ui (replaces the upstream "Antora default UI / MPL-2.0"
  boilerplate)
• `.card-grid` CSS for landing-page card grids (unused on this site
  in this PR — the existing index.adoc with the Key Features /
  Supported Model Families content stays untouched)
• Mobile responsive (hamburger menu from upstream preserved)

The change in this repo

One line in `docs/antora-playbook.yml`:

```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
  ```

Pinned to `v1.1.1` (not floating on `latest`) so transformers builds
are reproducible. Bumping the pin is a one-line follow-up PR whenever
`skainet-docs-ui` ships a new minor.

Out of scope for this PR

• Rewriting the landing page with a Diátaxis card-grid. The transformers
  landing has substantial real content (Key Features table, Supported
  Model Families table) and shouldn't be replaced. A future PR could
  add a card-grid SECTION above or below the existing content.
• 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.