docs: use ucp_version macro for all versioned spec and schema references

[damaz91]

Description

Replaces literal absolute links and hardcoded version references with versioned {{ ucp_version }} macros across documentation and specification files.

This resolves link checker failures in CI and ensures consistency by dynamically resolving versions in the built documentation.

Changes

• Replaced hardcoded versioned URLs (e.g., https://ucp.dev/2026-04-08/...) and version fields in examples with {{ ucp_version }} in:
  • docs/documentation/core-concepts.md
  • docs/specification/checkout-mcp.md
  • docs/specification/checkout-rest.md
  • docs/specification/checkout.md
  • docs/specification/overview.md
  • docs/specification/split-payments.md
  • docs/specification/identity-linking.md
  • docs/specification/permalink.md
• Fixed a markdownlint MD034 (bare URL) style issue in docs/specification/order-mcp.md.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Resolves blockages in PR #709 caused by pre-existing broken links in draft specs.

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

N/A