PR review plugin update docs on use-case and sdk workflow

[juanmichelini]

We've ported the plugin to the extensions repo: https://github.com/OpenHands/extensions/blob/main/plugins/pr-review/scripts/agent_script.py
But we still haven't deleted these scripts (prompt.py, agent_script.py) in the SDK repo: https://github.com/OpenHands/software-agent-sdk/blob/main/examples/03_github_workflows/02_pr_review/agent_script.py
This creates an issue in the long run: we end up maintaining two different versions of the same thing, and we are not allowing downstream users to use our best-performing PR review plugins (if they load stuff directly from extensions repo)

We want to:

1 - Delete the scripts from the SDK repo and directly load the plugins in the GH action (see issue OpenHands/software-agent-sdk#2282) and
2 - update all documentation related to the PR-review (one use-case, one SDK workflow (this issue).