Document Delivery API content type schemas in OpenAPI (CMS 18.0)

[lauraneto]

📋 Description

Adds a new article documenting the opt-in GenerateContentTypeSchemas option introduced in Umbraco 18 (CMS PR #22666). When enabled, the Delivery API OpenAPI document emits a schema per Document Type, Element Type, and Media Type, with discriminated unions on contentType and mediaType.

The article covers:

• The default behaviour and the opt-in shape.
• When to enable the feature (typed client generation, content-model discovery without Backoffice access).
• The configuration snippet.
• An example consuming the schemas with hey-api.
• A note on OpenAPI 3.1 polymorphism support in client generators.

Also updates the Content Delivery API landing page and the v18 SUMMARY.md with the new entry.

📎 Related Issues (if applicable)

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

CMS 18.0

Deadline (if relevant)

When possible, after the CMS 18.0 beta release.

[github-actions]

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[sofietoft]

The changes from this PR are applied to the new CMS structure here: #8061