Rewrite incremental migration guide

The /docs/incremental-migration page is now structured around vertical and horizontal migration strategies, with an animated diagram that shows each one in action.

• Reframed the page around incremental migration benefits, disadvantages of one-time migrations, and three strategies: vertical (one integration layer at a time across procedures), horizontal (one procedure end-to-end), and hybrid.
• Added an animated grid that plays vertical fill first, pauses, then horizontal fill so the difference is visible at a glance.