ci: add Docusaurus website build step to CI to catch broken links before merge

[sonukapoor]

Problem

Broken links in the website docs are only caught when Docusaurus builds in production. PR #548 merged with stale links to remediation-strategy.md that caused the production build to fail and required a follow-up hotfix (#549).

Solution

Add a CI step that runs npm run build in the website/ directory whenever docs files change. This catches broken links, missing files, and MDX syntax errors before they reach main.

Suggested implementation

Add a job to .github/workflows/ci.yml that:

1. Runs only when website/** files change (path filter)
2. Installs website dependencies (npm ci in website/)
3. Runs npm run build in website/
4. Fails the PR if the build fails