[Docs] Update structured output doc for V1

[russellb]

This should go in after #17085 as it includes references to that new feature.

---

Structured output works differerently in V1 than V0. Update this doc to
reflect V1 since that is now our default. The differences include:

• the backends available are different
• request-level backend choice is no longer supported
• whitespace_pattern is not supported
• structural_tag is new as of [V1] Add structural_tag support using xgrammar #17085

Signed-off-by: Russell Bryant [email protected]

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀