docs: add preview environments section to contributing guide

[inkeep]

Summary

Adds documentation for the new PR preview environments feature introduced in #2681.

Changes

• Added a new "Preview Environments" section to the contributing guide (agents-docs/content/community/contributing/overview.mdx)
• Documents the automatic full-stack preview environments that are created for internal PRs
• Includes preview URL patterns (https://pr-<PR number>-api.preview.inkeep.com and https://pr-<PR number>-ui.preview.inkeep.com)
• Explains the lifecycle (provisioned on PR open/sync, torn down on PR close)
• Notes that smoke tests run automatically to verify previews are healthy

Related

• Triggered by ci: provision PR preview environments in Railway #2681 (ci: provision PR preview environments in Railway)

[vercel]

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Project	Deployment	Actions	Updated (UTC)
agents-api	Ready	Preview, Comment	Mar 18, 2026 9:34pm
agents-docs	Ready	Preview, Comment	Mar 18, 2026 9:34pm
agents-manage-ui	Ready	Preview, Comment	Mar 18, 2026 9:34pm

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