Add public readiness docs for Python SDK

[negillett]

Summary

• Add TRADEMARK.md and link trademark policy in CONTRIBUTING.md.
• Expand README.md with install, verify, test, release, support, and CI badge.
• Align with Stage 1 public repository readiness checklist.

Test plan

• See repo-specific gates in checkpoint (docs-only where applicable).

Review

• Documentation-only diff; no blocking findings after internal review.

[cursor]

PR Summary

Low Risk
Docs-only changes (README/CONTRIBUTING plus new TRADEMARK.md) with no runtime or API impact; risk is limited to potential confusion if instructions/links are incorrect.

Overview
Adds public-facing repo readiness documentation: expands README.md with CI badge plus install/verify/test/coverage, release/signing, documentation hub, and support guidance.

Introduces a new TRADEMARK.md policy and links it from CONTRIBUTING.md/README.md, clarifying that Apache-2.0 does not grant trademark rights and pointing contributors/users to the trademark terms.

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