Summary

Updates the Pipelines documentation to reflect the pipeline → stream field rename in Wrangler pipeline binding configuration, matching the workers-sdk change in cloudflare/workers-sdk#13860.

• Updates the Wrangler config example in the "Writing to streams" page to use the new stream field
• Adds a deprecation note explaining the old pipeline field is still accepted but will emit a warning
• Adds a changelog entry with before/after config examples and migration guidance

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.