docs: update links to archived narwhal repository

[mlegner]

Description

I noticed that various places in the documentation still link to the archived MystenLabs/narwhal repository. Where applicable, I've updated the link to point towards https://github.com/MystenLabs/sui/tree/main/narwhal instead.

Test Plan

Only documentation changes, no tests required.

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[mlegner]

@randall-Mysten Should I update this PR to get it merged?

[randall-Mysten]

@randall-Mysten Should I update this PR to get it merged?

Yes, please. I fixed the merge conflict. These updates will show up on the docs site after you merge this PR.