docs: add sample walkthrough prerequisites

[FanouZeng-TT]

Description

The Python sample walkthrough says the reference server is "included in this repository," but every server command points to a sibling checkout:

uv sync --directory ../samples/rest/python/server/

A standalone conformance checkout contains no samples directory, and pyproject.toml also resolves ucp-sdk from ../python-sdk. Following the walkthrough without both sibling repositories therefore fails before the sample server can start.

Fix: identify the separate samples repository, document the required sibling layout, and add clone commands for both samples and python-sdk before syncing the environments.

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

N/A

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. (The documented sibling paths were checked against the three repository layouts.)
• 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. (Not applicable; no schema or model changes.)

Screenshots / Logs (if applicable)

documented sibling layout: samples/server and python-sdk pyproject paths found
ruff check / ruff format --check: passed
pre-commit: 12 hooks passed