docs: add CarbonAds to right sidebar

[mroderick]

Add CarbonAds to the right sidebar of the VitePress documentation site
using VitePress's built-in support — no plugins or custom components needed.

• VitePress renders the ad below the outline, pushed to the bottom of
  the aside by a flex spacer
• The Carbon script loads lazily only at desktop widths (hidden on mobile)
• Auto-refreshes on page navigation via _carbonads.refresh()
• Uses the same CE7D453M serve ID and sinonjsorg placement as the
  old Jekyll site

The old Jekyll site had CarbonAds via _includes/carbonads.html, but
this was never ported to the new VitePress site.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.40%. Comparing base (c1b92bf) to head (1b19081).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2729   +/-   ##
=======================================
  Coverage   97.40%   97.40%           
=======================================
  Files          39       39           
  Lines        1810     1810           
=======================================
  Hits         1763     1763           
  Misses         47       47           

Flag	Coverage Δ
unit	97.40% <ø> (ø)

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