ci(site): drop stale workflow triggers

[h3l1o5]

What changed

Two stale entries in the Site workflow's push triggers:

• docs/images/architecture.png — the architecture diagram was removed in 1d01112 ("docs: drop outdated architecture diagram from README"), so this path can never match again. It was also never mirrored in the pull_request paths.
• feat/website-site in branches — that feature branch has been merged (docs: 還原未經竄改的官方 Apache-2.0 LICENSE #7), so only plugin needs to trigger the workflow.

No job/step changes — trigger cleanup only.

How was this verified

Confirmed docs/images/ only contains logo.png, and YAML still parses. Workflow behavior on plugin pushes and PRs is unchanged.

• cortex-vec: ./scripts/run-checks.sh passes (if you touched Python)
• Website: node --test site/tests/*.test.mjs passes (if you touched site/)

Checklist

• Target branch is plugin
• Commit messages follow Conventional Commits
• Relevant docs (README / CHANGELOG) are updated (if needed)