#538 Fix XY docs anchors and match the current marketing banner

Fixes ENG-13558

Fix four chart-page jump links whose fragments dropped punctuation retained by heading IDs. Both bar-chart links now use #column-charts-(vertical-bars) instead of #column-charts-vertical-bars; line and contour jump links also retain their headings' period and commas. Add a site-wide check that authored in-page links resolve to rendered targets.

Match the current Reflex marketing announcement across all 75 XY docs pages: “Build vs. Buy for Internal Software,” its article URL, layered opal artwork, responsive spacing and wrapping, and release-scoped dismissal shared through local storage. Preserve the shared banner body and close event, and document the contract.

Validation:

• Docs suite: 144 passed, 3 production-only checks skipped in the default run.
• Production Chromium: desktop/mobile layout, exact copy and destination, reduced motion, cross-tab/reload dismissal, and blocked-storage fallback.
• Production sitemap, Markdown, HTML-route, and link validators; all 75 prerendered pages checked for the new title, URL, and release.
• Full-worktree pre-commit hooks, Ruff lint, and Ruff formatting.

Summary by CodeRabbit

• New Features
  • Added a responsive Build vs. Buy announcement banner to the documentation site, with artwork, an accessible close control, and dismissal remembered across reloads and tabs. The banner remains dismissible when browser storage is unavailable.
• Bug Fixes
  • Corrected chart-page links so they navigate to the intended sections.
  • Improved validation of in-page documentation links to catch links that do not resolve to a heading.