Add Miren Cloud docs section with subdomain guide

Miren Cloud is currently a single page buried in the Features sidebar. As we ship subdomains (and eventually more Cloud features), it needs its own top-level sidebar section.

Proposed structure:

- Miren Cloud
  - Overview (current working-with-miren-cloud content)
  - Subdomains (new — claim flow, base domains, assigning to clusters)

The subdomain page should cover the flow the blog post (MIR-816) links to: claiming a run.garden or miren.app subdomain, assigning it to a cluster, and deploying an app to it.

Ship alongside MIR-817 — doesn't need to be gated behind labs, just needs to be live before the blog post goes out.