ci(docs): unpin vp and move docs task cache settings under cache

[wan9chi]

#2814 pinned vp to 1.0.0-rc.0 for docs builds, because that release rejects task cache settings under cache and the new vite-task rejects them at the top level. 1.0.0-rc.1 ships the new format and is now npm latest.

• Remove the version: 1.0.0-rc.0 pin from setup-vp in .github/actions/deploy-docs/action.yml and .github/workflows/build-docs-fork-preview.yml, so docs builds use the latest release again.
• Move the build:site settings in docs/vite.config.ts under cache.

This PR's staging-deploy check builds the docs with 1.0.0-rc.1 and the converted config.

[cloudflare-workers-and-pages]

🚀 Deploying Preview to Cloudflare 🚀

Preview URL: https://chore-docs-task-cache-format-viteplus-dev.voidzero-docs.workers.dev (commit 17d0df7)

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Status	Deployment URL	Commit	Updated (UTC)	See this deployment's details
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[github-actions]

✅ Staging deployment successful!

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