docs: Add Cloud CDN and caching concept page

[developerjamiu]

Adds a new Cloud concept page, Content delivery and caching (cloud_docs/concepts/cdn.md), documenting the recently launched CDN feature.

The page covers:

• How caching works: the three cache layers (browser, CDN edge, server) and what the common Cache-Control patterns do at each.
• Enabling caching via cacheControlFactory, with the recommended public, max-age=60, s-maxage=86400 header.
• Verifying caching with the age response header (GET, not HEAD).
• Cost and limits: no separate charge, nothing to turn on, per-app isolation, custom domains covered.

Technical claims were checked against the framework source at current main, the cloud repo (E2E deploy test, CLI command surface), and a live cache-opted-in deployment. A few platform facts would benefit from cloud-team confirmation on review:

• Responses that set a cookie are never cached.
• HEAD requests are never cached.
• No usage limits and no separate charge.
• The deploy purge covers custom domains, and a purge failure never fails the deploy (freshness falls back to the CDN lifetime).
• The cleared cache refills per edge location.

Planned follow-ups in separate PRs: correct the default-caching claims on the framework Flutter web page (they predate the current uniform no-cache, private default), and add a one-line note to the Deployments page that a successful deploy clears the CDN cache.

[Zfinix]

LGTM 🚀