Add docs for gas fee refunds

[sketsdever]

Changes:

• Add a new page in both the builder and protect sections which describes the new gas fee refunds feature
• Update multiplexing instructions to include eth_sendBundle (which now supports multiplexing)
• Update Protect overview and quick start guides to mention gas fee refunds in addition to MEV refunds
• Update the eth_sendBundle API spec to explain smart bundles

Follow ups:

• Add API spec for flashbots_getSbundleStats in a separate PR

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Name	Status	Preview	Comments	Updated (UTC)
flashbots-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 8, 2024 9:00am

[bertmiller]

reviewed, i didn't look for nits but mostly lgtm with two comments

[bertmiller]

I thought we were defaulting to msg.sender unless there was an override?

[sketsdever]

That's true for protect (tx.origin not msg.sender to be precise). For anything sent directly to the bundle relay it's the signer because signer is always provided and this allows for separating eg. bot address from refund recipient which affords more privacy.