docs(examples): add Python client and usage examples

[FenjuFu]

Summary

• What changed? Adds examples/python/ — a dependency-light (requests-only) SkillHubClient plus runnable examples covering search → resolve → download → publish (and star/rate) against the REST API — and links it from the README Documentation sections (EN + ZH).
• Why is this needed? A real sample of recent stargazers skews heavily Python (~481) vs Java (~77), a mismatch with SkillHub's Java backend. There is currently no Python on-ramp, so Python-leaning visitors have no code-level way in. This gives them one and doubles as a reference SDK seed.

Addresses #701 (which also proposes evaluating a full published pip SDK as a follow-up).

What's included

• skillhub_client.py — SkillHubClient wrapping public + authenticated + CLI-compat endpoints; unwraps the {code,msg,data} envelope and raises SkillHubError on non-zero codes.
• example_usage.py — runnable: python example_usage.py (search/inspect/download) and python example_usage.py publish <zip> <namespace>.
• README.md — setup, quick start, endpoint coverage table.
• requirements.txt — just requests.

Endpoints are taken from document/docs/04-developer/api (public.md / authenticated.md / cli-compat.md).

Validation

• Backend tests passed
• Frontend typecheck/build passed
• OpenAPI SDK regenerated or checked when API contracts changed
• Smoke test run when relevant

Examples/docs only — no backend, API, or build surface changed. Locally verified python -m py_compile on both modules and an offline smoke test (client construction, Bearer/X-Request-Id headers, SKILLHUB_URL env resolution, rate() score validation) all pass. Live end-to-end calls were not run here (no running instance), but every endpoint maps to the documented API.

Risk

• User-facing impact: new examples/ directory + two README bullets; nothing existing changed.
• Deployment or migration impact: none.
• Rollback approach: revert the commit.

Notes

• Related issue: [Feature] 评估提供 Python SDK / Python 调用示例 #701
• Follow-up: maintainers to decide on a full published pip SDK (packaging/versioning/CI) — could be OpenAPI-generated to reduce maintenance.