Add HIP-904 docs: Frictionless Airdrop

[theekrystallee]

HIP-904 brings frictionless airdrops with senders able to create token associations for receivers at transfer. It eliminates the need for pre-association. It also supports pending token transfers for accounts that do not wish to receive unsolicited tokens, allowing receivers to claim or reject these airdrops. This will reduce user friction, enhance token association workflows, and prevent spam by making the user's experience as good as it can be with fungible and non-fungible tokens.

https://hips.hedera.com/hip/hip-904

Tasks

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1. Add HIP-904: maxAutoAssociation & sender pays & admin key support for contracts #701
   documentation enhancement +2
   
2. Add HIP-904: TokenReject #702
   +0
   
3. Add HIP-904: Airdrop/Claim/Cancel #703
   p1 tokens +2