feat(developer-portal): add Stellar provider docs (window..stellar)

[wabicai]

• developer-portal Provider API 新增 Stellar 链入口（window..stellar）
• 补齐中英文文档：Getting Started / Signing / API Reference\n- 方法签名与返回值对齐 Stellar Wallets Kit PR #80（ KitActions：getAddress / getNetwork / signTransaction / signAuthEntry / signMessage）\n

[revan-zhang]

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[wabicai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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