Add documentation for SSR on new svelte kit

docs/svelte-kit.md

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+# Usage in Svelte Kit
+
+> The solution is *heavily* inspired by https://github.com/sveltekit-i18n/lib/issues/94#issuecomment-1247942697.
+> 
+> All kudos to @jonsaw.
+> This is merely an adaptation to `svelte-i18n`.
+
+- Support for SSR
+- Example with user preference set by [`Accept-Language`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language) on the server and [`navigator.language`](https://developer.mozilla.org/en-US/docs/Web/API/Navigator/languages) on the frontend.
+
+## 1. Setup svelte-i18n
+
+The first thing we need to do is to setup regular `svelte-i18n`.
+
+```typescript
+// src/lib/i18n/index.ts
+import { browser } from '$app/environment'
+import { init, register } from 'svelte-i18n'
+
+const defaultLocale = 'en'
+
+register('en', () => import('./locales/en.json'))
+register('de', () => import('./locales/de.json'))
+
+init({
+	fallbackLocale: defaultLocale,
+	initialLocale: browser ? window.navigator.language : defaultLocale,
+})
+```
+
+## 2. Setup the hook
+
+For SSR we need to tell the the server what language is being used. This could use cookies or the accept-language header for example.
+The easiest way to set the locale in the [server hook](https://kit.svelte.dev/docs/hooks#server-hooks)
+
+```typescript
+// hooks.server.ts
+import type { Handle } from '@sveltejs/kit'
+import { locale } from 'svelte-i18n'
+
+export const handle: Handle = async ({ event, resolve }) => {
+	const lang = event.request.headers.get('accept-language')?.split(',')[0]
+	if (lang) {
+		locale.set(lang)
+	}
+	return resolve(event)
+}
+```
+
+## 3. Setup the layout
+
+On the frontend we need to set the language too, for this we could again use cookies, or in this example: `window.navigator.language`.
+
+
+```typescript
+// +layout.ts
+import { browser } from '$app/environment'
+import '$lib/i18n' // Import to initialize. Important :)
+import { locale, waitLocale } from 'svelte-i18n'
+import type { LayoutLoad } from './$types'
+
+export const load: LayoutLoad = async () => {
+	if (browser) {
+		locale.set(window.navigator.language)
+	}
+	await waitLocale()
+}
+```
+
+```svelte
+<!-- +layout.svelte -->
+<slot />
+```
+
+# 4. Use the library as normal
+
+```svelte
+<script lang="ts">
+	import { _ } from 'svelte-i18n'
+</script>
+
+<h1>Welcome to SvelteKit</h1>
+<p>Visit <a href="https://kit.svelte.dev">kit.svelte.dev</a> to read the documentation</p>
+
+{$_('my.translation.key')}
+```