docs: add Wally endpoint guide

[adityabatra072]

Summary

Adds a Wally custom endpoint guide under the LibreChat YAML AI endpoints docs.

The example uses:

• WALLY_API_KEY
• https://inference.runanywhere.ai/v1
• glm-5.3-flash, deepseek-v4.1-flash and mimo-v2.6-pro as the default models
• models.fetch: true so LibreChat can load the live model list from /v1/models

Testing

• pnpm lint:prettier
• pnpm typecheck
• pnpm lint
• pnpm test

[vercel]

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