ci: show the docs review environment only when docs change

What does this MR do?

The manual review-docs-deploy job appears on every merge request as "Can be manually deployed to review/mr-<iid>", which doesn't say it's a docs preview, and it shows up even when an MR touches no docs.

• Renames the environment to docs-review/mr-<iid> in both review-docs-deploy and review-docs-cleanup, so the MR widget reads "Can be manually deployed to docs-review/mr-<iid>" and the environments group under a docs-review folder.
• Gives .review-docs its own rules, so both review jobs are added only to merge request pipelines that change docs/**/*. They stay manual.

The preview URL is unchanged. Existing review/mr-<iid> environments stop on their own through auto_stop_in: 2 weeks.

How to verify

• On this MR (no docs changes), the pipeline has no review-docs-deploy job and the widget shows no deploy prompt.
• On an MR that changes a page under docs/, the widget shows "Can be manually deployed to docs-review/mr-<iid>". Running the job deploys to the same docs.gitlab.com/upstream-review-mr-cli-<iid>/cli URL.

🤖 Generated with Claude Code