Better emphasize benefits of Narwhal/Tusk in docs.sui.io Learn section

[Clay-Mysten]

What are the existing URLs containing the issue, if any?
https://docs.sui.io/learn/about-sui
https://docs.sui.io/learn/sui-compared
https://docs.sui.io/learn/how-sui-works

How would you describe the issue? (What needs changing?)
We introduced Narwhal months ago but truly make little reference to its benefits given the docs above precede the consensus protocol's launch. See feedback indicating as much in:
https://discord.com/channels/916379725201563759/955861929346355290

We should fix that. Thanks to @sblackshear for reporting it. See other related fixes to terminology in:
#2672

Where should this information land if new? (Learn, Build, Explore, Contribute.)
See the URLs above. Here is the target for linking:
https://github.com/MystenLabs/narwhal

What changes are needed to support this new feature or product? (A simple overview, detailed procedures?)
N/T need more links in.

Who is the audience for that information? (App developers, investors, creators, validators, someone else?)
System implementers

What precisely must they do with it? (Configure a new service? Process some output?)
Use the consensus protocol for more complex (shared) transactions.

[Clay-Mysten]

Progress in:
#3765