ci: enable docs preview domain

[yomna-shousha]

Summary

• keep capnweb.com as the production custom domain
• add pr.capnweb.com as a preview-enabled custom domain route
• keep bare pr.capnweb.com disabled while allowing preview URLs like https://<preview-name>.pr.capnweb.com

Verification

• tested the same config shape on a disposable Worker using testpr.yomnashousha.com and pr.testpr.yomnashousha.com
• npx wrangler deploy reported pr.testpr.yomnashousha.com (custom domain) [disabled, previews: enabled]
• npx wrangler preview --name smoke2 returned preview_urls: ["https://smoke2.pr.testpr.yomnashousha.com"]

Follow-up

This must be applied by npx wrangler deploy -c packages/docs/wrangler.jsonc; wrangler preview does not create/update custom domain routes. The existing Deploy Docs workflow runs that command on main, so this should be provisioned after merge when the docs deploy runs.

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Latest commit: 21587d5

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[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/cloudflare/capnweb@262

npm i https://pkg.pr.new/cloudflare/capnweb/capnweb-validate@262

commit: 21587d5

[dimitropoulos]

thanks - added in a021985