docs: add Brainiall LLM Gateway provider guide

[fasuizu-br]

Summary

• Adds a provider guide for connecting Brainiall LLM Gateway to Open WebUI
• Brainiall is an OpenAI-compatible gateway providing 113+ models from 17 providers (Anthropic, DeepSeek, Meta, Mistral, Amazon, Qwen, etc.) through a single endpoint backed by AWS Bedrock
• Guide covers Admin Panel setup, Docker Compose configuration, available models table, and troubleshooting

Details

New file: docs/tutorials/integrations/llm-providers/brainiall.md

The guide follows the existing community contribution format (warning banner, prerequisites, step-by-step setup, Docker Compose example, troubleshooting section) matching the structure of other provider guides in the same directory.

Test plan

• Verify the page renders correctly in Docusaurus
• Verify sidebar auto-generates the new entry under LLM Providers
• Verify all links are valid

[pr-validator-bot]

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