docs: document typed params in page props

[RazinShafayet2007]

Closes #14066

This PR documents the typed params prop that was introduced in #13999.

Previously, the routing docs showed how to access typed data in +page.svelte via PageProps, but did not mention that params is also available and typed based on the route parameters. This made the feature difficult to discover for users working with parameterised routes.

This change:

• Updates the +page.svelte routing example to destructure params from $props()
• Demonstrates usage of params in a [slug] route
• Updates the $types section to explain that PageProps includes typed route parameters

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15284-svelte.vercel.app/

[dummdidumm]

thank you