docs: add UplinkSwitch custom endpoint guide

[czkendra]

Summary

Adds an UplinkSwitch custom endpoint guide under the LibreChat YAML AI endpoints docs, with its sidebar entry and a provider icon registered the same way as the other endpoint pages.

The example uses:

• UPLINKSWITCH_API_KEY
• https://api.uplinkswitch.com/v1
• claude-sonnet-5-5, claude-opus-5-5, gpt-6.1-sol, gemini-3.1-pro-preview, grok-4.6 and deepseek-v4-pro-0813 as the default models
• models.fetch: true so LibreChat can load the live model list from /v1/models

English only. The locale copies come from the translation workflow.

Testing

• pnpm build passes; the page renders at /docs/configuration/librechat_yaml/ai_endpoints/uplinkswitch
• pnpm lint:prettier passes
• pnpm lint and pnpm typecheck pass
• curl -fsSL https://api.uplinkswitch.com/v1/catalog | jq -e '[.data[].id] | contains(["claude-sonnet-5-5","claude-opus-5-5","gpt-6.1-sol","gemini-3.1-pro-preview","grok-4.6","deepseek-v4-pro-0813"])' returns true

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