docs: add links to checkout samples and SDKs

[omsaisudarshan108]

Summary

• Add a short “Samples and SDKs for checkout integrations” section to the core concepts doc.
• Link to the UCP samples repo and Python SDK for multi-merchant checkout flows.

Testing

• Not run (docs-only change)

Additional Notes

• None

[omsaisudarshan108]

Summary

• Add four roadmap-aligned doc stubs: Agent Checkout Quickstart, Cart and Basket Building, Loyalty and Member Benefits, Post‑Order Management.
• Wire the new pages into site navigation and llms output.

Testing

• Not run (docs-only change)

Additional Notes

• These are starter outlines intended to evolve with the roadmap.

[jingyli]

Thank you for your contribution to the Universal Commerce Protocol!

While we appreciate your enthusiasm for UCP, it looks like this PR has some overlapping changes as #15 and due to similar reasons mentioned there we will be declining changes that are specifically related to new guides.