docs: explain how dutch auctions work

[conorsch]

We landed Dutch Auction support (#4196) recently, but the docs/guide don't reference it anywhere. We should write some explanation about how one can interact with dutch auctions and add those to docs/guide/. At the very least, we want explain how this works via pcli; later on we could reference how it works in different UIs, if relevant.

[conorsch]

We're hoping to add some uniswap v3 compat to at least pcli in the near future; after landing that work would be a good time to revisit the dex legibility docs.