ci: strengthen documentation validation pipeline

[jankiluitel]

Summary

This PR strengthens the existing CI pipeline for the ThothTech Documentation Website by modernising GitHub Actions and adding automated production-build validation.

Changes

• Upgraded actions/checkout from v2 to v4.
• Upgraded actions/setup-node from v1 to v4.
• Added npm dependency caching.
• Replaced npm install with npm ci for reproducible CI installations.
• Preserved the existing Linelint, Prettier, and Vale checks.
• Added npm run build to validate Astro/MDX compilation and internal links.

Motivation

The existing CI pipeline validates formatting and prose quality but does not run the Astro production build.

This means documentation changes could potentially pass CI while still containing build errors, invalid MDX, or broken internal links.

Adding the production build provides an additional automated quality gate for documentation contributions, including the SplashKit documentation workflow.

Testing

• npm ci completed successfully.
• npm run format:check passed.
• npm run build completed successfully.
• Temporarily introduced an invalid internal link in a SplashKit Usage Examples page to verify the new validation.
• Confirmed that the build correctly failed and identified the invalid link.
• Removed the temporary test change and confirmed the build passed again.
• Confirmed no documentation content changes are included in this PR.

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[SteveDala]

LGTM

[Rhinoatron]

Amazing work. The CI changes look focused and useful. Updating the GitHub Actions versions, switching to npm ci, adding npm caching, and keeping the existing Linelint, Prettier, and Vale checks all make sense. Adding npm run build is also a good improvement because it validates the actual Astro/MDX documentation build and can catch issues that formatting or prose checks would miss. I also checked for overlap with other PRs and did not find another current PR making this same CI/build-validation change. Approved.