[3.x] Add Layers documentation

[claudiodekker]

This documents the new Layers feature (inertiajs/inertia#3236): pages that render on top of the page they were opened from, which is how you build modals, slideovers, and wizards in Inertia.

Rather than adding a standalone "Layers" section, each piece is documented on the page where you'd naturally look for it:

• Responses: returning a layer with ->layer(base: ..., key: ...), and closing one with Inertia::close().
• Redirects: marking a redirect as a detour with ->interstitial(), so a pending layer survives a full-page interruption like password confirmation.
• Manual visits: the client side. router.layer(), local layers, useLayer(), making requests inside a layer, and layer events.
• Client-side setup: the layer option (your app-wide shell component) and the loading option (the placeholder shown while a directly-opened layer's base page is fetched).
• The protocol: the layer, close, and interstitial page-object fields, close instructions, and detours, for anyone implementing a backend adapter.
• Testing: hasLayer(), layerBase(), and isClose() assertions.

All client-side examples come in Vue, React, and Svelte flavors, and the sections cross-link so you can start from whichever end you're coming from.

Companion PRs

• inertia-laravel: ->layer(), Inertia::close(), ->interstitial(), and testing assertions: [3.x] Add support for Layers inertia-laravel#903
• Client implementation (Vue, React, Svelte, SSR): [3.x] Add Layers (Modals / Dialogs / Slideovers) inertia#3236
• Kitchen sink demos: Add Layers demos demo-v3#24