ci: probe docs preview main cache fallback

[wan9chi]

Summary

• Add a workflow-only comment to deploy-docs-preview.yml.
• Add a markdown-only probe comment to docs/guide/cache.md.
• This is a cache fallback and invalidation probe for docs preview caching.

Result: PR can replay main cache

Confirmed in Deploy Docs Preview run 28633592137, job 84915309878:

• PR key requested: vite-task-docs-Linux-X64-pr-2026-92362796f7d05cdaafba0037c5cb0fe0cb16eeef
• Restored from main fallback: vite-task-docs-Linux-X64-main-7575a9532b2f6c45cfdaa459a4c66c054dc53cf9
• vitepress build result: cache hit, replaying
• Vite Task summary: 1/2 cache hit (50%), 23.72s saved
• Saved a new PR-scoped cache key after the run: vite-task-docs-Linux-X64-pr-2026-92362796f7d05cdaafba0037c5cb0fe0cb16eeef

Result: markdown change misses cache

Confirmed in Deploy Docs Preview run 28633701063, job 84915631020:

• PR key requested: vite-task-docs-Linux-X64-pr-2026-cddbc65b2eca6bfa7c9ecd36e99ad23d191037f4
• Restored previous PR cache: vite-task-docs-Linux-X64-pr-2026-92362796f7d05cdaafba0037c5cb0fe0cb16eeef
• vitepress build result: cache miss: 'guide/cache.md' modified, executing
• Vite Task summary: 0/2 cache hit (0%)
• Saved a new PR-scoped cache key after the miss: vite-task-docs-Linux-X64-pr-2026-cddbc65b2eca6bfa7c9ecd36e99ad23d191037f4

Validation

• Latest PR checks completed with 11 passed, 16 skipped, 2 neutral, 0 pending, 0 failed.

[netlify]

✅ Deploy Preview for viteplus-preview canceled.

Name	Link
🔨 Latest commit	4ce1d2c
🔍 Latest deploy log	https://app.netlify.com/projects/viteplus-preview/deploys/6a475a18060781000860aa68

[github-actions]

✅ Staging deployment successful!

Preview: https://viteplus-staging.void.app/
Commit: cddbc65

[fengmk2]

merge #2028 first, then we can check in this pr.

[wan9chi]

@fengmk2 this pr is only to check cache reuse in "Deploy Docs Preview", and it's not meant to be merged, so it isn't blocked by #2028.