docs: clarify local site build and serve steps

[FanouZeng-TT]

Description

The documentation says ./scripts/build_local.sh "will build the site and
start a local server," but the script only builds local_preview/ and prints
the command that the user must run next:

echo "To serve the fully built site (with versioning):"
echo "  python3 -m http.server 8000 -d $OUTPUT_DIR"

There is no executable python3 -m http.server or mkdocs serve invocation in
the script. The README already provides the manual server command immediately
below, so the current sentence incorrectly promises a running server when the
build finishes.

Fix: describe build and serve as two steps: the script builds the site, and
the command below starts the local server.

Category (Required)

• Core Protocol: Protocol specification changes. (Requires Technical Council approval)
• Governance/Contributing: Governance or contribution process changes. (Requires Governance Council approval)
• Capability: Capability specification changes. (Requires Maintainer approval)
• Documentation: Documentation updates. (Requires Maintainer approval)
• Infrastructure: Build, CI, tooling, or release changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency, compatibility, or routine maintenance changes. (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. (Requires Maintainer approval)
• Community Health (.github): Repository-wide health files and configuration. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide.
• 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)

Static script check confirms no executable server command; full example validators
and validator unit tests pass with ucp-schema on PATH. All remaining
pre-commit hooks (cspell, whitespace, YAML, shebang, Ruff, markdownlint,
stylelint, Prettier) and git diff --check pass.