chore(docs): adopt new @theholocron/astro-config defineConfig

[iamnewton]

Summary

Switch docs/astro.config.ts from the temporary vanilla astro/config defineConfig to the updated @theholocron/astro-config defineConfig now that it's published (v7.18.2). The new API accepts docs.name, docs.github, and docs.sidebar directly — no importMetaUrl or federated path assumptions.

Test plan

• tsc --noEmit passes in docs/
• pnpm --filter @theholocron/themes-site build succeeds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

@@            Coverage Diff            @@
##              main       #46   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines           46        46           
  Branches        13        13           
=========================================
  Hits            46        46           

Components	Coverage Δ
docs-theme	100.00% <ø> (ø)

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