fix(ci): align job triggers with website change detection
This PR fixes pipeline failures that occurred when pushing to main without website changes. The pages job was running on every push but waiting on docs_to_pdf, which only triggered on website changes - causing the pipeline to fail. See issue #564 (closed).
All CI jobs now share the same trigger conditions, only running when relevant files change (website/, ci/, .gitlab-ci.yml, yarn.lock). A trigger matrix has been documented in ci/README.md.
Also cleans up the lockfile situation - removes the package-lock.json and ensures yarn.lock is tracked.