v0.15.2

Docs: subdomain collection hubs

Follow-up to the v0.15.1 fix. The docs now explain that a collection deployed to a subdomain renders its root page with the collection's own index template (and context), just like its index on the main domain:

• The Trust Center reference gains a "Deploying to a subdomain" section — serve the trust hub from trust.example.com, optionally paired with private = true for content behind Cloudflare Access.
• The Configuration reference notes that the subdomain root uses the collection's index template (e.g. trust-index.html).

No code changes.