Add note about transfer function to Unsupported Calls page

[amiecorso]

What changed? Why?
This PR adds a note about Solidity's builtin transfer function and how this can't be used to send ETH to Base Accounts (or likely most smart contract wallets).

Original context came from this slack thread, and the failure to be able to withdraw ETH from the WETH smart contract due to its use of this function.

Notes to reviewers

How has it been tested?
Local review of the hosted docs

[cb-heimdall]

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[cb-heimdall]

Review Error for youssefea @ 2025-07-18 17:27:23 UTC
User cannot review their own commit