docs: add v2026-08-25 protocol release announcement to ucp.dev

[carolinerg1]

Description

This PR adds the official release announcement for protocol version v2026-08-25 to the ucp.dev announcements page (docs/documentation/announcements.md).

It also adds the v2026-08-25 specification index reference under 'lllstxt' documentation section to mkdocs.yml

Verification & Testing

• Checked local rendering via mkdocs serve and verified link formatting.
• Validated doc examples via python3 scripts/validate_examples.py --schema-base source/schemas --scaffolds scripts/scaffolds --file docs/documentation/announcements.md.
• Formatted markdown via ./.venv/bin/mdformat docs/documentation/announcements.md.
• Checked YAML syntax via ./.venv/bin/yamllint mkdocs.yml.

Category (Required)

Please select one or more categories that apply to this change.

• 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

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)