docs(nats): update gateway auth docs for internal_authn flow

[HJianBo]

Summary

• refine NATS gateway auth docs based on review comments
• clarify the two authentication paths (internal_authn and authentication) and fallback order
• split internal auth docs into Token / NKey / JWT sections with NATS Server references
• add JWT ACL explanation and a claims example
• sync updates across Chinese, English, and Japanese docs

Files

• zh_CN/gateway/nats.md
• en_US/gateway/nats.md
• ja_JP/gateway/nats.md

[github-actions]

🔗 Link Check Results

Summary

Status	Count
🔍 Total	8578
✅ Successful	8346
⏳ Timeouts	0
🔀 Redirected	0
👻 Excluded	216
❓ Unknown	0
🚫 Errors	14
⛔ Unsupported	2

Errors per input

Errors in en_US/deploy/cluster/create-cluster.md

• [ERROR] https://etcd.io/docs/latest/install/ | Error (cached)

Errors in en_US/emqx-ai/rtc-services/volcengine-rtc/quick-start.md

• [412] https://www.bilibili.com/video/BV1P2WTzBEu4/ | Error (cached)

Errors in en_US/getting-started/feature-comparison.md

• [ERROR] https://console.cloud.google.com/marketplace/product/emq-public-380104/emqx-cloudpay-as-you-go | Error (cached)
• [403] https://marketplace.microsoft.com/en-us/product/saas/emqtechnologiesincorporated1678779968155.emqx_cloud?tab=Overview | Error (cached)

Errors in ja_JP/deploy/cluster/create-cluster.md

• [ERROR] https://etcd.io/docs/latest/install/ | Error (cached)

Errors in ja_JP/emqx-ai/rtc-services/volcengine-rtc/quick-start.md

• [412] https://www.bilibili.com/video/BV1P2WTzBEu4/ | Error (cached)

Errors in ja_JP/performance/benchmark-emqtt-bench.md

• [ERROR] https://www.xmeter.net/ | Error (cached)

Errors in zh_CN/deploy/cluster/create-cluster.md

• [ERROR] https://etcd.io/docs/latest/install/ | Error (cached)

Errors in zh_CN/emqx-ai/rtc-services/volcengine-rtc/quick-start.md

• [412] https://www.bilibili.com/video/BV1P2WTzBEu4/ | Error (cached)

Errors in zh_CN/faq/concept.md

• [403] https://medium.com/@flespi/http-vs-mqtt-performance-tests-f9adde693b5f | Error (cached)

Errors in zh_CN/getting-started/feature-comparison.md

• [ERROR] https://console.cloud.google.com/marketplace/product/emq-public-380104/emqx-cloudpay-as-you-go | Error (cached)
• [403] https://marketplace.microsoft.com/en-us/product/saas/emqtechnologiesincorporated1678779968155.emqx_cloud?tab=Overview | Error (cached)

Errors in zh_CN/index.md

• [404] https://www.emqx.com/zh/blog/a-data-driven-solution-for-logistics-asset-tracking-and-maintenance | Error (cached)

Errors in zh_CN/support/technical-support.md

• [412] https://space.bilibili.com/522222081 | Error (cached)

Full Github Actions output