docs: add bot platform capability guide

[Pr1p]

Summary

Refs #3894.

Adds synchronized English and Chinese documentation for the eight current bot platforms, including a source-derived capability matrix, provider setup guidance, security boundaries, limitations, and a repeatable live-validation checklist. This is a documentation-only change; it does not change runtime behavior.

Verification

• ASF license header check: passed
• Focused Bot bridge tests: 100/100 passed
• WeChat-focused tests: 14/14 passed
• git diff --check: passed
• Feishu live smoke test on a mainland China account: direct text reply, group @Maka reply, content preservation, and post-restart group reply passed
• Discord live smoke test from a mainland China network via the Discord web client: group @Maka round trip and exact reply content passed; a system-level TUN route was required for the Gateway WebSocket
• Telegram live smoke test from a mainland China network: direct-message round trip and exact reply content passed
• WeChat is source-verified but not live-verified. The current iLink adapter does not retain inbound context_token and sends an empty token for outbound replies, so the documentation records this implementation risk instead of claiming live support.
• The full runtime suite is not a clean local pass on this Windows host because unrelated shell/symlink tests fail; no runtime source files are changed by this PR.

No credentials, user IDs, or raw provider payloads are included. Restart recovery, duplicate suppression, group behavior where not tested, and scheduled delivery remain untested in the live smoke entries.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope:

Codex was used for source review, documentation drafting, platform smoke-test recording, and verification.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No