feat: Refresh docs theme styling and icon set

[elibosley]

Summary

• replace several default Docusaurus theme icons with Tabler-based overrides and update the iframe back arrow
• refine dark theme, footer, and DocSearch styling to better match the Unraid docs look and update the dark theme-color metadata
• render footer copyright years dynamically across localized footers and ignore the local .gstack/ directory

Testing

• pnpm run test:run
• pnpm run lint
  • passes, but Docusaurus reports pre-existing broken links to warn/ in translated pages and one pre-existing broken anchor under SMART reports

Summary by CodeRabbit

• New Features

  • Many new UI icons with a unified icon wrapper; dynamic footer shows the current year; new floating feedback widget; responsive embed accepts optional class overrides.

• Style

  • Dark-mode meta/color tweak and refined menu/dropdown caret and search/external-link visuals; several theme CSS improvements for visual consistency.

• Chores

  • Added Tabler Icons React package; updated ignore rules.

• Documentation

  • Added validation guidance file.