docs: rewrite the Named Routes page

[skirtles-code]

There are several problems I was trying to address:

1. Named routes are used in earlier pages, particularly the Nested Routes and Programmatic Navigation pages, which use them quite heavily.
2. The list of advantages comes too soon. A reader will struggle to understand that list until they have seen the example.
3. Advantage 2 on the list mentions encoding/decoding. As far as I can tell, that should just be encoding, as named routes don't impact decoding.
4. Advantage 4 stops mid-sentence. Bypassing path ranking (e.g. to display a ).
5. The linked example is for Vue 2.

While I've reworded and reordered much of the page, the material covered should be the same.

I've tweaked the example to use name: 'profile', rather than name: 'user'. This was to avoid the name coinciding with the /user/ prefix on the path. These kinds of naming coincidences can (in general) lead to beginners misunderstanding what is significant and what isn't, and it seemed unnecessary to risk any confusion here.

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