add sidebar topics plugin, update sidebar links

ChunkyCapybararequested to merge
feature/sidebar-topics into master
  • Adds a separate sidebar for each of the sections (Start Here, TODO, Legacy) using starlight-sidebar-topics plugin
  • Legacy section mirrors the legacy challenge sidebar
  • URLs in legacy section also mirror the legacy challenge, so with a redirect to /legacy/* any existing links from the legacy site should open the correct page in the new site

CleanShot 2025-09-29 at 20.46.07@2x.png

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