chore: update outdated screenshots, outdated button refs

[james-westbrook]

Summary

Updates multiple screenshot images in the docs to the new UI from v0.11.x, and replaces a few old references to web search being under the + button.

Related issue or discussion

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers