fix(docs): pin zod to v3 to fix docs build (#1223)

<!-- acp:session_id=session-227bb086-4be4-456d-9d48-9d0d9cbabe9c
source=#1190 last_action=2026-04-06T19:30:00Z retry_count=0 -->

## Summary

- Pin `@astrojs/sitemap` to `~3.6.1` (last version using zod v3) and pin
`zod` to `3.25.76` via npm overrides
- `@astrojs/sitemap@3.7.x` introduced a zod v4 dependency that conflicts
with `astro@5.x` (which requires zod v3), causing the build to crash
with `Cannot read properties of undefined (reading '_zod')`

## Test plan

- [x] `npm run build` in `docs/` completes successfully
- [ ] CI docs workflow passes

Closes #1190

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Co-authored-by: Ambient Code Bot <bot@ambient-code.local>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

docs/package-lock.json

@@ -20,6 +20,8 @@
     "rehype-mermaid": "^3.0.0"
   },
   "overrides": {
-    "lodash-es": ">=4.18.0"
+    "lodash-es": ">=4.18.0",
+    "@astrojs/sitemap": "~3.6.1",
+    "zod": "3.25.76"
   }
 }