docs: add step for adding font links in svelte:head to sveltekit installation guide

[Mef45]

When trying to implement unplugin-fonts into my SvelteKit project today, I followed the provided instructions but continually experienced issues with my fonts not displaying.

After some troubleshooting, I discovered that due to SvelteKit not using transformIndexHtml vite process, the automatic addition of font links from vite.config.js(ts) based on the injectTo parameter isn't possible.

The workaround solution for this is to add the following code into +layout.svelte (or another preferred location):

<script>
  import { links } from 'unplugin-fonts/head'
</script>

<svelte:head>
  {#each links as link}
    <link {...link?.attrs || {}} />
  {/each}
</svelte:head>

This snippet is currently utilized in an existing SvelteKit example, however, it's not explicitly mentioned in the instructions that this is a necessary step.

In this PR, I have included mention of this code snippet in the instructions for adding the plugin to a SvelteKit project. I believe that this will prevent similar issues from occurring amongst other users.

[stafyniaksacha]

Thanks for your contribution @Mef45 !