docs: new FAQ explaining functionality of gas manager

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New Documentation / Existing Documentation
Explain:

the way the gas manager functions, is we actually front the gas for you and then charge you in USD on your alchemy account with the equivalent amount.

You will need to be on a paying tier to sponsor gas on mainnets.

Flag that this is similar to a paymaster.

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If new documentation, a clear summary of what needs to be added
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