Docs: Add a small guide on database migrations

[AgarwalPragy]

Summary

Litestar is a database-agnostic micro-framework.

Still, both beginner and seasoned devs would first look at the official Litestar docs to see if there is a recommended way of setting up the database. This expectation becomes more relevant since Litestar does provide plugins to work with SQLAlchemy.

While the current docs include the database setup, it doesn't include migrations.

A small guide on how to setup alembic migrations while using the various SQLAlchemy plugins that Litestar provides will be awesome!

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[Yacobolo]

This would be much appreciated. Just read through the docs and was missing this 🔥