Use npx next fallback in docs build

[spMohanty]

Summary

Follow-up fix to make docs build checks robust in environments without a global next binary.

What changed

• Updated Makefile docs build step so cd website && npm run build falls back to npx next build.
• Kept existing behavior by running node scripts/generate-llmstxt.mjs after fallback build.
• Leaves existing local/dependency-first path unchanged when next is available from local dependencies.

Validation

• make docs-build no longer requires a globally installed next CLI in PATH.
• Existing generated docs JSON artifacts remain local-only working-tree noise from earlier ad-hoc runs and are intentionally not part of this commit.