docs: add Linkrra custom endpoint guide

[jimmyplup]

Summary

Adds a Linkrra custom endpoint guide under the LibreChat YAML AI endpoints docs, following the existing provider-page conventions (same shape as the NEAR AI Cloud page).

Linkrra provides an OpenAI-compatible inference API for open-weight models. Disclosure: I work on Linkrra.

The example uses:

• LINKRRA_API_KEY
• https://pool.linkrra.com/v1
• qwen3-coder-30b and glm-4.7-flash as the default models (the IDs currently returned by https://pool.linkrra.com/v1/models?format=openrouter)

Files:

• content/docs/configuration/librechat_yaml/ai_endpoints/linkrra.mdx (English only; left to the translation workflow)
• content/docs/configuration/librechat_yaml/ai_endpoints/meta.json sidebar entry
• components/icons/providers/linkrra.tsx, registered in components/icons/providers/index.ts and lib/icons.tsx (monochrome currentColor version of the Linkrra mark)

Testing

• prettier --check on the changed files
• fumadocs-mdx && tsc --noEmit
• eslint --max-warnings 0 on the icon and both registries
• vitest run (37 files, 421 tests passed)
• node scripts/sync-config-version.mjs --check
• Sent streaming chat completion requests for both default models to the endpoint with the base URL and Bearer key from the example.

Not done: a full pnpm build, and I did not run the YAML inside a LibreChat instance.

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