Clarify IPFS node/gateway usage in retrieve quickstart

[vmx]

I've read through the retrieve quickstart and didn't find it as clear as it could be. I like the distinction between IPFS node vs. gateway and trusted vs. verified. Though in the later section it's not always clear to me to which category something belongs.

More concretely:

• Fetching the CID with Kubo: I would welcome a sentence that states that as it's a IPFS node it's of course verified.
• Verified retrieval with Helia Verified Fetch: It's not clear whether it's using an IPFS node or a gateway (it's using both)
• Fetching the CID with an IPFS Gateway: Mention that those are trusted and not verified.