Docs: add types to `navigating`

[theetrain]

Describe the problem

From the navigating docs, I cannot tell what are the types for navigating.to or navigating.complete https://svelte.dev/docs/kit/$app-state#navigating.

Describe the proposed solution

Let's find a way to make the types for these modules clearer within the documentation.

Alternatives considered

No response

Importance

would make my life easier

Additional Information

Relates to:

• site: goto parameter documentation not appearing on kit.svelte.dev svelte.dev#651
• Link digests between tutorial, guides, and types; and feature summaries svelte.dev#747