Add Subscription Items API documentation

[Benzilla]

Add Subscription Items API documentation

Context

Zoneless is adding Subscription Item support through #159, with corresponding Node SDK support tracked separately.

The public API reference should document the Subscription Item object and every public endpoint Zoneless supports.

Blocked by: #159 and #160

What to build

Add Subscription Items to the Zoneless API documentation.

Use Stripe's Subscription Items documentation as the structural reference:
https://docs.stripe.com/api/subscription_items

Follow the API docs conventions in AGENTS.md.

Acceptance criteria

• Add a Subscription Items section to the API reference
• Document the Subscription Item object and its supported fields
• Document every public Subscription Item endpoint implemented by Zoneless
• Include parameters, response examples, and Node SDK examples
• Node examples use the real zoneless.subscriptionItems.* SDK methods
• Add the resource to the appropriate docs navigation/catalog
• npx nx test docs passes

Out of scope

• Documenting Stripe fields or behaviour Zoneless does not support
• Adding or changing API endpoints
• Changing the Node SDK

Please follow AGENTS.md and CONTRIBUTING.md.

[Benzilla]

Closed by #176