feat: rewrite and complete the migration guide from custom arch

[illright]

Background

This is part of the ongoing effort to complete the most important pages before the 2.1 release. This page was marked as a WIP, and contained advice that encouraged early division by features, which runs contrary to the current understanding of an optimal migration path.

Changelog

1. Rewrite the guide for migration from a custom architecture
2. Rename the page from "migration from legacy" to "migration from a custom architecture" to be more respectful

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