ci: validate tutorial builds and document contribution checks

[FenjuFu]

Documentation builds currently run only after changes reach main. Add pull-request CI that installs the locked dependencies and runs the production VitePress build, retaining its internal Markdown page-link validation. Also verify the published learning-assistant index contains valid Chinese and English entries and matches the freshly generated index.

Use the same Node.js 24, npm ci, and npm test commands for Pages deployment. Document the required contributor checks and their limits, and add a SECURITY.md that directs private reports to the existing iFLYTEK community security process.

Validation:

• npm ci and npm test pass locally on Node.js 24.16.0; 413 published index entries validated.
• The new Linux CI run passes.
• Confirmed VitePress rejects a deliberately broken local page link.
• Confirmed the index validator rejects an empty index, a missing language, missing headings/content, and a stale published index; all temporary fixtures were removed.
• git diff --check passes.

The separate Vercel preview status requests deployment authorization from the Vercel team; the documentation CI itself is green.

[vercel]

@FenjuFu is attempting to deploy a commit to the Fenju's projects Team on Vercel.

A member of the Team first needs to authorize it.