Add deprecated banner to Splice docs

[danielporterda]

Summary

Adds a global deprecated-site banner to the Splice Sphinx docs published at docs.sync.global.

The banner is injected through the shared Sphinx layout so normal docs pages, search, and generated index pages all receive the notice. The only generated HTML files without the banner in the smoke build are the two sphinx-reredirects redirect stubs, which immediately redirect to sv_helm.html, where the banner is present.

The visual treatment now reuses the Canton Network docs brand palette from digital-asset/cf-docs (#734BE2, #5C3DB8, and a light lavender surface) rather than a generic warning-orange style.

Design artifacts

• Figma flow diagram: https://www.figma.com/board/CuPjtik4q5Ru3xAhZ38RCd?utm_source=codex&utm_content=edit_in_figjam&oai_id=&request_id=d309b53b-fc1e-4b46-955c-617fef55746e
• High-fidelity Figma mockup: https://www.figma.com/design/O6R2PSKSq5pHbXyaFgoiua
• Website-accurate captured mockup frame: https://www.figma.com/design/O6R2PSKSq5pHbXyaFgoiua?node-id=8-2

Screenshots

Desktop overview page

Shows the deprecated banner below the existing breadcrumbs/edit area and above the page heading, using the Canton Network docs purple/lavender brand treatment.

Mobile overview page

Shows the banner stacked for narrow viewports with the call-to-action still visible before the deprecated page content.

Validation

• git diff --check
• Local Sphinx smoke build with a temporary Python venv: sphinx-build -b html -q docs/src /tmp/splice-docs-banner-smoke
• Verified generated output contains the banner and stylesheet on 83 of 85 generated HTML files; the two exceptions are redirect-only stubs: sv_operator/bootstrap.html and sv_operator/onboarding.html.
• Verified index.html, search.html, genindex.html, and overview/overview.html include the banner and stylesheet.
• Captured desktop and mobile screenshots from the rendered local Sphinx output using headless Chromium.

Note: I attempted to use the repo direnv/Nix shell first. The worktree .envrc needed flakes enabled, and after setting NIX_CONFIG='extra-experimental-features = nix-command flakes', nix print-dev-env did not finish materializing the dev shell in a practical window. The smoke build above uses the same Sphinx config path but does not run the repo's full sbt docs/bundle, so it reports existing warnings for generated Daml API and metrics pages that are normally produced by the full bundle task.

[isegall-da]

@danielporterda
Who decided and approved this deprecation? I am not aware of any such decision.