Update Cloudflare imageService docs for custom image service support

[adamchal]

Clarify that compile mode uses a configured custom image.service when defined (running its getURL()/getHTMLAttributes() hooks during prerendering), and that custom mode now also generates optimized assets at build time via the service's transform() hook, without bundling Sharp when it is not needed.

See withastro/astro#17099.

Description (required)

Two mode descriptions were out of date:

• compile previously always used the adapter's internal workerd image service. It now uses the configured image.service if one is defined (running its custom getURL() and getHTMLAttributes() hooks while prerendering), and falls back to the internal dependencies only when no custom service is set.
• custom previously ran only at runtime. It now also generates optimized image assets at build time for prerendered routes (the service's transform() hook runs at build), and Sharp is no longer bundled into the worker when the configured service does not need it.

The edits keep the existing bullet structure, reuse the existing Image Options link, and preserve the existing workerd runtime caveat on custom.

Related issues & labels (optional)

• Related: Respect custom image services for Cloudflare build-time generation (compile + custom) astro#17099
• Suggested label: improve or update documentation

Discord username: adam.chal

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