docs: Lumenfall image provider tutorial

[mikewirth]

Hi Open WebUI!
I'm the founder of Lumenfall.ai, an AI media gateway offering many different image models behind an OpenAI-compatible API.
We have no markup on provider prices and also unlimited free models.

This pull request adds a docs page describing how to add Lumenfall as an image generation / editing provider to Open WebUI.

I tested the integration and the steps. The doc includes a screenshot.

[pr-validator-bot]

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