docs: fix broken links in draft specification

[mercael91]

Description

Replaces literal absolute links in the draft specification with versioned {{ ucp_version }} macros.

This resolves link checker failures in CI that were uncovered when the link checker was updated to be scheme-insensitive (matching hostnames). Previously, these https://ucp.dev/2026-04-08/... links were hardcoded with a literal date instead of using the versioned macro.

Changes

• docs/documentation/core-concepts.md: Replace 8 instances of https://ucp.dev/2026-04-08/ with https://ucp.dev/{{ ucp_version }}/ to match the pattern used in all other specification files.

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 #710
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

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[damaz91]

Thanks, let's follow up on #710 directly