Enable anonymous telemetry defaults

[suguanYang]

Summary

• enable self-hosted anonymous telemetry by default with TELEMETRY_ENABLED=true
• configure the PostHog ingestion host/project token through self-hosted env defaults
• document the anonymous installation id, opt-out flag, and privacy boundary in English and Chinese docs
• keep dashboard NEXT_PUBLIC_POSTHOG_* analytics separate from server-side self-hosted telemetry

Notes

• This pairs with Add anonymous self-hosted telemetry client knowhere#142, which adds the runtime telemetry client and startup/heartbeat events.
• The project token is a PostHog public ingestion token, not an API secret, but the runtime still avoids logging it.

Tests

• bash -n scripts/entrypoint.sh
• git diff --check

[suguanYang]

do not expose posthog in the document, and I didnt understand why we need so many telemetry env vars, like TELEMETRY_POSTHOG_HOST, didnt this should be configed for a fixed knowhere prod api?