docs: add vite.config.ts option for SvelteKit 2.62.0+ path aliases

[techreign]

What was wrong

The SvelteKit and manual installation pages only showed configuring path aliases in svelte.config.js. Since @sveltejs/kit 2.62.0 (sveltejs/kit#15944) the SvelteKit config, including alias, can be passed directly to the sveltekit() Vite plugin in vite.config.ts, and when you do that svelte.config.js is ignored. The docs never mentioned this.

What changed

Following @ieedan's suggestion on the issue, both pages now lead with the vite.config.ts example and add the note: "When using SvelteKit versions older than 2.62.0, the aliases configuration will be in your svelte.config.js file instead", keeping the original svelte.config.js snippet below it.

• docs/content/installation/sveltekit.md
• docs/content/installation/manual.md

The vite.config.ts example uses the flat option shape (sveltekit({ alias }), no kit wrapper), matching the SvelteKit configuration docs.

Verification

pnpm -F docs build:content   # velite build finished, 0 warnings
pnpm -F docs check           # svelte-check: 0 errors, 0 warnings
pnpm -F docs build:svelte    # built successfully

Fixes #2750

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