(fix): Document cors_origins and anonymous configuration options

[catplat]

Why

Closes #{ISSUE_NUMBER}

What's changed

• Adds a new Configuration options section documenting the anonymous and cors_origins keys, with a reference YAML example and an options table.
• Rewrites the routing step to present sub-path and subdomain routes as explicit, labeled choices rather than a single example — and adds a warning explaining why sub-path routing is required for browser clients (CORS), with a note on the anti-pattern of hardcoding domain names in cors_origins.

The equivalent change has also been made to the Upsun Flex topic.

Where are changes

https://fixed.docs.upsun.com/add-services/mercure.html

Updates are for:

• platform (sites/platform templates)
• upsun (sites/upsun templates)

[github-actions]

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See the changed pages:

Upsun Fixed docs

• https://fix-mercure-cors-config-rwrr7rq-ucq44jg6ofare.eu-5.platformsh.site/add-services/mercure.html

Upsun docs