docs: Add manual Alembic database migration guide

[silentoplayz]

This change adds a new, comprehensive Docusaurus page that provides instructions on how to manually apply Alembic database migrations. This is in response to a GitHub discussion where users were encountering issues with automatic migrations after updating the application.

Related - #843, open-webui/open-webui#19487

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[Classic298]

@silentoplayz lets merge this for now and refine it later