docs: add Manifest provider to providers page

[SebConejo]

Description

The providers documentation page is missing a section for Manifest, an open-source LLM router. Manifest is being added to models.dev (anomalyco/models.dev#1686) and needs a corresponding docs section in providers.mdx so users know how to connect it.

Proposed solution

Add a Manifest section to packages/web/src/content/docs/providers.mdx following the same format as the OpenRouter section (setup steps, API key, model selection, self-hosted note).

[github-actions]

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What needs to be fixed:

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[github-actions]

This issue has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

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